As others have said, this has nothing to do with polish. On the other hand, I don't agree that it's bad game design or anything like that. Rather, I think it is simply a fact of life when dealing with newer technology and creating professional content.

Look at the game "Doom" by id software - there were literally hundreds of thousands of custom maps created by third parties, and the total conversions probably numbered in at least the hundreds. Doom was a simple game, and it is very easy to create 2d sprites, low-res textures, and 2.1d maps.

Compare that with "Quake" and the number don't go down too much - there were still a lot of user-created modifications, but many of those dealt with the new scripting system and far fewer included custom art because 3d models are more difficult to create than sprites (consider that you can turn a 3d model into a sprite but not easily vice versa).

Then come along games like "Serious Sam" that have much higher quality maps, textures, and models. The amount of user created content significantly drops, and the portion of that considered to be of decent quality is even lower. This isn't because serious sam is more difficult to make modifications for, but simply because making truly 3d maps that look nice is a difficult skill, and properly blending multiple layers of textures to get a nice detail effect is not trivial.

I tried making a simple lava room in a map I was working on, and gave up after I opened one of the original maps to find literally hundreds of different lights in 10+ different colors used to make the glowing lava effect I wanted. It isn't their fault that it takes that much to make perfect lighting, it's just that they spent the time to put in the extra effort to make their game look better. Compare their lava to something like "Unreal Tournament 2004" and you can see that the textures themselves aren't too different, but in unreal, the level designers didn't put in the extra effort and the difference is obvious. Unreal lava looks acceptable, but the lava in Serious Sam just has that extra something. If you hold yourself to the highest standards, things just become more difficult. If you don't, you deliver an inferior product. Either choice is fine for third party freeware, but it's hard to choose the latter once you've tasted quality.