And some of them realistically can't. Every other commercial game engine is developed for the studio first; Cry, Source, Unreal etc. These engines were made for, well, Far Cry, Half-Life 2, Unreal Tournament. The studio saw returns for engine development in the sales of games, then they said "We could probably further monetize the work we've already done if we license the engine and SDK out to third parties."

Unity on the other hand is trying to have the Autodesk/Adobe business model of "We have a free student or hobbyist tier, and then a commercial license that's $100,000 per minute per seat." The thing is, Autodesk and Adobe really don't have realistic competitors in their market sectors. Unity very much does. Unity competes directly with GameMaker Studio, Godot, Unreal, Source 2 among others, the development of which are either directly supported by the sales of first party titles (or are outright FOSS projects in the case of Godot). So Unity has to set their prices to compete in that market, without the support of first party game sales.

You can see how that's working out for them.