I used to do a lot of frssh water spearfishing - it is not as easy as it looks. At one extreme you have 2-3 ft of viz and the fish check you out long before you see them. At the other extreme you have 30 ft viz and the fish never get close enough to shoot (large fish like Walleys, Sauger etc, not counting pan fish like bluegill, Crappie, etc that are too small to shoot).
In some cases you end up having to stir the viz up a bit or back into your own silt blind.
Fish like salmon, trout and northern Pike are very skittish and you rarely see them. Samlon for the most part also run very deep, and that produces its own challenges.
A purist will spearfish with freediving equipment only, but even with scuba it is not as easy as it looks givent the above conditions and the noise of a regulator. Then there is the predator factor. When you are one fish seem to pick up on it.
That is not to say a you cannot clean up and fill your limit, but hou per hour, dollar per dollar the hook and line fishermen will be more consitent more of the time. Taht makes sense as they can cover a lot more water in a lot less time. For example, it takes time to set up and hop off the boat on a prospective point to spear Walleye and they will either be there or they won't and if not it takes time to get out, move and get back in compared to a boat that will zip from point to point or troll from point to point and in the process cover much more water.
25 years ago in the Missouri river system around Lake Oahe we would routinely bag Walleyes in the 4-6 pound range and pass on the larger 8-10 pounders as the smaller ones were better eating. Now in the same waters 4-6 pound Walleye are rare - not because of the divers spearfishing (the numbers have alwyas been small) but rather because of the exponential growth in fishermen who have always out numbered divers by hundreds to one, and by the introduction of Salmon that competed for the same feeder fish as the Walley. Biologists insisted that the Walleyes fed shallow and the Salmon fed deep, but if they had asked the divers, we'd have told them the shiners moved bac and forth and were not separate shallow and deep populations. The end result was that the Salmon devestated the baitfish popluation with fantastic growth rates - until the bait fish ran out and then both Salmon and Walleye basically subsisted on a meager diet with slow growth.
The reality was that the plummet in size and numbers of the walleye population there was due primarily to huge increases in fishermen - and ignorant biologists driven by a tourist industry hot to maintain popular bag limits of large fish and also develop a new market in salmon fishermen. The result has been size limits with a minimum and maximum size limit that makes spearfishing very difficult (try to spear the fish - but be sure to measure it first.) That often leads to the banning of spearfishing as if you shoot it and it is too large or too small you can't exactly release it.
On the other hand as both a diver and a park ranger in the late 1980's I often saw the bottom littered with dead fish after tournaments where fisherment would constnatly cull the smaller fish out of the live well as they caught larger ones, even though the survival rate was very low. The survival rate is not much better for over or undersized fish unless very good cathc and release techniques are used (such as the fish never being lifted out of the water to remove the hook) as just the act of being caught tires adn stresses the fish to the extreme and starts a downward spiral resulting in death in many lakes and streams where food stocks are marginal due to poor mangement.
In that regard spearfishing is no worse, and infact is better as we don't pretend the fish will survive being caught and then being subjected to a photo op with bubba and his drunk fishing buddies.
But as posted above, fishermen have the numbers, the money, the noisiest lobbists to ensure they continue unihibited.
And if you spear walleye in lake oahe and they see a dive flag on a point for more than 10 minutes, you will have a herd of boats overhead trying to catch the fish you are trying to shoot and a few years ago they conveniently got the dive flag laws changed to allow then to run you over as long as they do it with an electric trolling motor - that you of course can barely hear, so be real careful when you surface.