Well, JJ2+ is a community product into which countless hours of free labour have been put, so I think your analysis is a little off the mark there. But manually redrawing every single sprite frame is a lot of work that also requires some specific skills (which only rarely overlap with programming skills), and the fact of the matter is that it doesn't look like anyone here is willing or able to do it. So that leaves commissioning someone to do it, which requires money.

The other games you mention all have larger communities or are 3D, which makes them hard to compare. You should also consider that JJ2+ exists, and adds a ton of new features and fixes to the game, much like the custom doom engines and whatnot.

Algorithmic rescalers are nice up to a point, and it would be cool to see whether they look any good on JJ2, but plugging them into JJ2's graphics systems is not trivial. If someone who knows a lot about old-school graphics engines like JJ2's volunteers to take a stab at it, they're welcome to do so - but that hasn't been the case so far.