A lot of the reason behind why there isn't a lot of kitbashing is simple (and you already noted it)- there isn't a reason to do it because there are no rules for it in the game(s). Necromunda is a bit different in that you have access to a lot of options via the Trading Post, but for the mainline games like 40k or AoS, if the model doesn't have rules then you don't play with it. There are exceptions, but that would be the main reason that kitbashing isn't done for the majority of the gaming community.
Even in Necromunda, you'll face some difficulties in finding an in-game reason to kitbash. Yes, you can buy almost anything in the Trading Post, but you have some issues even with that; the "more boys or more toys" conundrum is always there, as is the obvious problem of real-world money and getting the most value out of your hobbying dollars. Is it really smart to spend X amount of RL money on a kit-bashed Orlock with a multi-melta when you may not ever use it beyond the one campaign? If you are playing with a dedicated, serious group of Necro addicts that will play either long campaigns or use older gangs when starting up a new campaign, then you may have a reason for the super-cool conversion. If, on the other hand, you play shorter campaigns or ones that frequently involve either new gangs or newer players, then you may never have the chance to use the conversion again. Those are some of the problems with kitbashing.
I kitbash entire new gangs. One of my on-going projects is a gang of Hive Brats. In case you don't know what a Hive Brat is, a bit of a history lesson. The predecessor to Necromunda was a game called "
Confrontation", the rules for which were published in a series of articles in White Dwarf magazine back in the early 90s. In Confrontation, one of the gang types were called Hive Brats: young members of rich noble houses who dressed flamboyantly and used exotic weapons. When Necromunda was published, the Hive Brats were replaced by the Spyrers (which, while still young nobles, were something completely different) and their flamboyant look was given to the Eldar Harlequins.
Anyways, I figured that young nobles getting together to venture into the Underhive to raise hell and maybe make a name for themselves are very much a likelihood in the world of Necromunda so I decided to build some... even though there are no rules for them. I decided to go with a more cyberpunk look rather than the flamboyant look that the original Hive Brat figures. I imagine that each individual gang of Hive Brats have their own unique look to identify them. Anyways... mine are a combination of Eldar Harlequin and Tau Pathfinder parts. It is proving to be something of a pain as not only do the Pathfinder legs need foot swaps but the Harlequin torsos are too small at the waist to properly fit on the legs and will require greenstuff, which I'm not very good at. Heads are also a problem. Human heads are larger than Eldar heads and typically look ridiculous when put on an Eldar body. There is also the issue of hair. Hair style is often part of a gangs' look so I want my Hive Brats have a particular hairdo... shaved bald on the side but left long on top, similar to a mohawk but rather than sticking up, it is allowed to fall to one side or else pulled back into a tail (a warhawk). Obviously, heads with such a specific hair style aren't going to be found. So... not only do I have to find bald human head (male and female) that fit on Eldar bodies but then I'm going to have to greenstuff hair onto them. Ugh!
(And I actually wonder why more people don't kitbash?

)
As for rules, that's easy. They're young nobles therefore they're rich and have access to more advanced technology. Thus, logically they "count as" Van Saar.
"Counts As" was much easier with original Necromunda. Each of the gangs essentially specialized in a specific trait:
- Orlock - Toughness
- Escher - Speed
- Goliath – Strength
- Van Saar - Tech
- Cawdor - Faith (Willpower)
- Delaque - Stealth
Simply pick which trait best suits the idea/theme of your new gang and voila, "counts as".
I'm also kitbashing military/security forces for the various Necromunda Guilds, even though there are no rules for them. There aren't even any rules for some of the Guilders themselves. We don't even know what an Air Guilder looks like, though I imagine long, flowing robes and lots of tubes. There isn't any art of an Electro Guilder either but there is at least something of a description.
Obviously, creating whole new gangs is not something most players are going to do. As you said, most players just want to play the game... and there's nothing wrong with that.
[Psychoanalytical Anti-GW Rant Cancelled.] It would just be encouraging to see some originality and creativity rather than just the same thing over and over. If nothing else, it would give me ideas and inspiration for my own kitsbashes.
What any of this has to do with anything, I don't know but....



