Cornwallis was based on a peninsula between the James and York rivers where they enter Chesapeake Bay.

There was an American army between them and any land route the reinforcements could take.

The short answer is they did, but it was two days too late!

Here's the longer answer. Cornwallis with his army was in Yorktown, on the banks of the Chesapeake. Washington and de Rochambeau were camped outside with with their army, building earthworks and generally prepping for a siege. Marching a relief army up wasn't a realistic option -- too slow* -- especially as the English still needed to solve the supply problem for the army inside. That left sea power.

The French had a fleet of 24 ships under Admiral de Grasse in the Carribbean that took a long and circuitous route to avoid getting spotted on their way to the Chesapeake. An English fleet of 15 ships under Admiral Graves was pursuing them, but arrived first (and didn't know it!) because they took the quick route. Both sides knew that the French and Americans could attack either New York or Yorktown, so Graves headed for New York, where he picked up four more ships and headed back in an attempt to capture and destroy a reinforcing French fleet (whose cargo included a bunch of heavy artillery for the siege) under Admiral Barras as well as his original quarry, de Grasse.

When the English got back to the Chesapeake, de Grasse and his ships were hanging out and refitting, with a good chunk of the crew and officers on land. de Grasse cast off without them and engaged the English.

After an indecisive fight that left both sides pretty beat up and the English minus one ship (it was too leaky and had to be scuttled) the French baited the English into chasing them away from the river for a few days before returning and linking up with Barras.

The now badly outnumbered English fleet headed back to New York to gather up reinforcements for the land and sea battles, but by the time they returned with a relief force it was two days past the surrender.

• there are about 400 miles between NY and Yorktown. Armies don't tend to go much faster than 20 miles a day. At that pace, you'd expect them to arrive in about a month. Given that it took about two months to organize a relief effort, a force wouldn't have arrived by land until sometime in November, a good month after the actual surrender!