Basically, the French Navy wasn't strong enough to attack England directly once war started.

When they concluded their alliance with the Americans, the French had 21 ships of the line at Brest and 12 at Toulon. The English had around 55, 22 more than the French. An attack through the channel would simply be unfeasible. Britannia ruled the waves, so there was simply no way France could land a force in England, and even if they did, that force would have been quickly defeated.

Despite this, the English cabinet still panicked and decided to keep up to half of the Royal Navy in home waters. They wanted to protect the Home Islands above all else. Lord Sandwich, the first Lord of the Admiralty, and Admiral Keppel, commander of the "Grand Fleet" that protected the British Isles, agreed that no diversions could be made.

The French and later the Spanish did plan to invade England, but their poorly thought-out plan failed before it even started, with thousands of soldiers dying of disease. After that, the French decided to send their navy to the Americas, especially the Caribbean and Boston. Since most of the Royal Navy was in England, this gave the French a fighting chance. The French fleet slipped through Keppel's Grand Fleet and set sail for the New World in May. Admiral Byron, with 13 ships, was sent to reinforce the British Army buy lack of decision within the cabinet and adverse weather slowed him down - he reached the United States in August.

The Spanish and French did try to take Gibaltrar, in what was called the Great Siege. But this operation makes apparent the weaknesses of their combined armada, which could not stop the English from reinforcing and supplying their garrison, despite numerical superiority in land and sea. The Siege ended in an English victory.

Other French attempts to attack the British failed, often misserably. The most well known among those is the Battle of Jersey, which, despite a succesful French landing, was a great English victory: there were around 70 English cassualties, compared with almost 600 French ones.

Simply put, the British position at the sea was too strong and an attack would be too costly in men and money to be feasible, while aiding the colonies would be easier, cheaper, and promised gains if victory was reached (mostly in the Caribbean).

Sources:

Robert Middlekauff, "The Glorious Cause" and Alan Taylor, "American Revolutions".