Some are, true. But the ones with lots of features, that everyone really wants to use—the ones that you can make games for multiple platforms, and that big development houses use—those... are not free.
It's much less than 75%—but then, you only get the engine, you don't also get the multimillion dollar award winning game to build on top of. If you make a mod for Skyrim—say, a new dungeon—you're adding that to the existing game and assets and dungeons and all that. You don't have to build everything from scratch, and you don't need your new content to be enough by itself to get people to buy it. A game with 1 dungeon, who would buy? But a mod that adds 1 dungeon to Skyrim, sure.
So yes, 75% is high, but the conversation should be, what cut should everyone receive, not, why does Bethesda deserve a cut at all, which most of the conversation actually is.
Well, I could say that the fact the assets already exist in the game that I paid for, then the assets have already been paid for whether there's a mod that makes use of them or not. The other factor is that the game assets the modder is using is only available to players who have purchased the game. The assets have been paid for once, what this system is asking that the assets be paid for more than once. Mind you, not paid for by the modder, but by the player who has already paid once for those assets. To expand that to an obvious conclusion; let's say there is a sword asset in the game I paid for. I pay for ten mods that make use of that same sword asset in different ways. Which means I have paid for that same sword eleven times. Which is free money to the developer for doing absolutely nothing beyond providing the initial game I already paid for.
Also, are they going to have different percentages based on amount of usage of game assets? So far I haven't seen an indication of that.
As I have said before, if Skyrim had been free then I could understand the position. In this case I feel it is in the best interest of the company and game to let the modders go forward without interference. But hey, that's just my opinion, it's their company and their game so they can do whatever they want.