I am not an historian, so please correct me if I'm wrong.

I assume you are talking about European exploration, because well, the natives of the West African coast did probably trade and go about. I believe there were several reasons:

• North Africa was part of the muslim world, and relations with Europe were not great. I believe the Jews were the only community with members in both the christian and muslim worlds, and thus able to trade. A christian captain would take great risks by navigating along a hostile coast, and remember in those days you had to resupply frequently.

• Which brings me to my second point, the Sahara. There probably wasn't much water and people along the coast south of Morocco, so supplies were probably a big problem.

• Navigation was another problem. You see, going south along the coast was quite easy as you say. And the winds are actually in your favor, as they also blow to the south. But in those days navigating against the winds was hard with the ships they had. That meant that to come back to Spain or Portugal, one had to go West to the Sargasso Sea, then come around to the Azores. But in those days, they didn't know that, because nobody had done it, and sailing far from the coast was very unreliable until sailors started using the astrolabe in the XVth century. So basically, sailing south along the coast was a one-way trip, which explains why nobody wanted to do it.

• Finally, there were plenty of legends, fantasies about monsters and whatnot. The southern limit of the known world was cape Bojador, in Morocco. There were stories of sandbanks and strong currents making it difficult, which were proven untrue, but for a long time nobody tried.

I used a lot of info from the French National Library website: http://expositions.bnf.fr/marine/arret/10-3.htm http://expositions.bnf.fr/marine/arret/10-32.htm

There's an article about the problems with cartography at the time: http://www.lecfc.fr/new/articles/216-article-6.pdf

you keep in sight of a coast... thats reasonable safe isnt it ?

Not really. If you ever read Homer's Odyssey, in the XII book, Circe instructs Odysseus (Ulysses) on how to pass between Scylla and Charybdis. These stood in a strait between two promontories:

they are so close together that there is not more than a bowshot between them [1]

On one side there was Scylla:

Of these two rocks the one reaches heaven and its peak is lost in a dark cloud. (...) Inside it Scylla sits and yelps with a voice that you might take to be that of a young hound, but in truth she is a dreadful monster and no one- not even a god- could face her without being terror-struck. She has twelve mis-shapen feet, and six necks of the most prodigious length; and at the end of each neck she has a frightful head with three rows of teeth in each (...) No ship ever yet got past her without losing some men, for she shoots out all her heads at once, and carries off a man in each mouth. [1]

One the other stood Charybdis:

You will find the other rocks lie lower (...) and under it lies the sucking whirlpool of Charybdis. Three times in the day does she vomit forth her waters, and three times she sucks them down again; see that you be not there when she is sucking, for if you are, Neptune himself could not save you. [1]

Nowadays, we know that this is referring to the straits of Messina. Charybdis would be a natural occurring whirlpool in the Northern portion of the strait and Scylla is thought to be a rocky shoal on the Calabrian side. The dangers of navigating this route created the Myth.

Same thing happened in Western Africa. To reach further South without known safe routes, a sailor would have to follow the coast and try to go through the obstacles that might appear. One of them, Cape Bojador in Western Africa, just South of the Canary Islands was apparently hard to surpass. The strong currents and changing winds turned a rather plain-looking point into a fearsome obstacle. The arid land and lost expeditions gave way to the Myths of monsters and boiling seas.

For their part, the Portuguese sea captains were reluctant to cross the southern threshold marked by Cape Bojador (about two-thirds of the way down modern Morocco’s long Atlantic coast), and with good reason. The last known expedition, by the Vivaldi brothers of Genoa in 1291, had never returned. It was widely feared that the very strong southern current that sweeps along the shore would frustrate any return. And to this day Cape Bojador marks a climatic, cultural and emotional frontier. For once Cape Noun is passed on the way south towards Bojador, all recognisable signs of Mediterranean life – trees, cultivation, farmland, villages, houses, man and goat – are gradually bleached out of the landscape (...) The shoreline is awesomely sterile, overlooked by wind-eroded cliffs, protected by reefs and with the tidal reach of the rocky shore everywhere presenting a razor-like surface. In addition the whole region is made even more dangerous and impenetrable by salty sea mists, a dense, muggy intensity of climate and erratic compass fluctuations. [2]

That's until Gil Eanes, a Portuguese explorer, found a route to safely navigate the Cape, finally opening the Western African coast to European explorers.

However, by 1434 one of the young squires of Prince Henry’s household, urged on by words of affection from his master, rather than threats, did manage to break this psychological frontier. Throughout the subsequent century of seafaring nothing halted the spread of Portuguese mariners across the oceans of the world, as Cape Bojador had. The breakthrough was the cumulative achievement of decades of unaccounted and unacknowledged work by shipwrights and observant sailors who had slowly transformed the traditional Arab-derived coastal craft of the Algarve into the lateen-rigged caravel. Together they created a craft strong enough to ride out oceanic storms but light enough to navigate estuaries and river mouths. It had the tactical ability to make use of the Atlantic winds and yet it could also be manned by a scratch crew of a dozen hands. [2]

That's considered a major breakthrough and it's even acclaimed in Fernando Pessoa's Mensagem (Portuguese poet book similar to the Lusiadas in the way it acclaims the Portuguese Discoveries).

To avoid the strong currents pushing the ships South, they used the Northern winds of the North Atlantic Gyre to sail back to Europe. They called it "volta do mar longo" (wiki link).

Sources:

[1] Homer's Odyssey, book XII (link)

[2] Barnaby Rogerson's The Last Crusaders: East, West, and the Battle for the Center of the World (link)

About the rest, I'm Portuguese and it's part of my cultural inheritance.

Edit: Added information.