It's worth remembering that not only did the Greeks know that the earth was spherical, but they also had a pretty accurate idea of how large it was. Eratosthenes calculated the circumference of the Earth to within about ten percent of its actual value during the second century BCE using trigonometry, and the distance that value gave was way greater than any ship of the time could possibly travel. The same was true up to Columbus' time. The reason that Columbus was willing to set out on his voyage is that he thought that the earth was much smaller than it actually is. If he hadn't bumped into Hispanola he and his crew would have starved.

it's really really really hard to do especially learning how to measure latitude and getting ships strong enough to survive the journey. I mean you might as well ask why the western Romans didn't sail directly to India since they knew it existed.

Also in the 1300s the Portuguese colonized some atlantic islands like the azores

Before you discount the Vikings, it is only fair that you discount, going in order of who was known to be first, i. e. Pytheas (who was one hell of a sailor in his day) the man who gave us the term Ultima Thule.