/u/tezoatlipoca has a great post exploring the mobility question. Related to that is that ships are, quite simply, really good at transporting heavy weights efficiently in a way that can't be done on land.
A 38-pounder gun and its carriage might weigh up to 3 tons, but once it was manhandled on deck, it was able to join a couple dozen of its mates, and a couple dozen slightly smaller 18- or 24-pounder guns on the upper gun deck, and a dozen or 18 12-pounder guns, on a typical 74-gun ship. That whole floating gun battery could move about (albeit slowly) with only wind power, whereas any of those guns on land would be nearly impossible to handle without a large team of horses, a large-wheeled carriage, and good firm roads.
This size disparity in artillery endured throughout WWII (and would likely endure today if the battleship hadn't become obsolescent) -- first-line battleships of the British, French, American and Japanese navies mounted 16"/406mm guns, with two Japanese battleships mounting up to 18.1"/460mm guns. The standard large artillery in the current U.S. Army and USMC is a 155mm howitzer, with plans to replace it with a slightly smaller in weight gun in the future for more mobility.
Artillery of a naval size just wasn't and isn't practicable for armies (although the German army, for example, deployed two 16.5"/419mm siege guns at the start of WWI). But those big land guns are very much the exception.
TL;DR - flip your question around - a land army didn't need as many cannon as a squad of naval ships due to mobility.
Land artillery back in the 1700s, 1800s was designed to be placed high on a vantage point where it could command a battlefield. Field mobility was a huge requirement - a fluke cavalry charge could put your battery placement in jeapordy, so gun crews would drill extensively to be able to hitch, relocate and unhitch in a relatively short time. Field mobility required large carriages with big spoked wheels to be able to struggle through the mud of the battlefield, and handle long marches over very rough roads.
The naval gun carriages of the time had relatively small wheels - essentially the carriage was only there to provide recoil and to run the guns in to reload and out to fire. Even on the smooth decks of a 74, the small wheeled carriage typically had to be handled by block and pulley. With enough elbow grease a field cannon could be manhandled by its crew for resighting - the cannon barrel was balanced pretty much over the axle of the wheels, so the crew would pick up the cannon and pivot like you would a wheel barrow.
Correspondingly to aim a naval gun - remember the truck would have 4 small wheels - you could use a long pole to pry the truck a few degrees left or right, but for aiming a gunner really only could change the elevation of the barrel and had to rely on whoever was piloting the ship to aim all guns in a broadside.
So essentially the answer to your question is mobility. Field guns were quite mobile. Naval guns not so, and they're tied to the ship in terms of being in one place and how they're aimed.
Its a bit of an extreme example, but here's really the best example that illustrates 1700s, 1800s field gun mobility. Thats a 12 lb cannon? For manhandling the actual gun and its carriage over obstacles you see it can be realistically done by 6-8 guys: https://youtu.be/7G-vQjBFU-4?t=138 Now imagine if you had a team of horses!