It seems to me that with British ability to take any point on land and with their naval supremacy, they’d be able to capture any munitions factories on land and prevent any smuggling of arms by sea.
This is a great question, and one the American Revolutionaries struggled with during the war. Before getting to the meat of the answer, we have to clear up a few misconceptions.
First, while the British had naval superiority and did mount a blockade of the US, they did not have anywhere near enough ships to patrol the entire coastline of North America. The only real way to close off a port was by occupying it or placing it under close blockade, which took up ships and manpower the British did not usually have available. The British captured every major port city in America at one point or another during the war, but did not hold them all at once. In particular, Boston stayed open for the entire war after the British evacuated in the spring of 1776, and it became the major port for French supply ships.
Even though the British could land anywhere, it doesn't mean that they should. The British Navy didn't have specialized landing craft like the Allies used on D-Day. It was possible to send small parties ashore in rowboats, but the safest way to disembark large numbers of troops and supplies was at a port. The British also had to keep logistical concerns in mind: any force that landed on the coast would need to protect its vulnerable supply lines, or try to "live off the land" seizing food from the civilian population on the march. Being that American militia could mass with surprising speed on any force that went into the American interior, the British wisely restricted their coastal raiding to mostly New England and New Jersey, where troops could either be quickly reinforced from or evacuated to the main base at New York.
Secondly, the Americans, or anyone else really, didn't have munitions factories. In the US, the small number of firearms produced under government contract were made by gunsmiths running small shops. American attempts and running a cannon foundry were mostly unsuccessful. Instead, the majority of firearms and cannons used by the Continental Army were either captured from the British or imported from France.
The British contracted the manufacture of different musket components - barrels, stocks, locks, etc - to various gunsmiths, then collected the pieces at central armories (most famously, the Tower of London). Armorers there would then piece the components together into working firearms.
The French had the most centralized system, giving the economy of whole towns, like Charlesville or St. Etienne, over to firearms production. Like the British, components would be made by various craftsman and then assembled at an armory, but it was all confined to the one town/region.
That all being said, I've written a little bit about the diplomatic and practical aspects of the Americans securing arms shipments from France here.
Sources:
Bodle, Wayne. The Valley Forge Winter: Civilians and Soldiers in War (2002).
Carp, E. Wayne*.* To Starve the Army at Pleasure: Continental Army Administration and American Political Culture, 1775-1783 (1984).
3). Fleming, Thomas. The Perils of Peace