I m not sure. Personally, Mamizou is one of my fav characters.
It does seem that the games we have do not tend to involve Mamizou but it will be too unkind to blame the game developers. We wouldnt have that many touhou games to play without them.
Realistically speaking, many of the developers are not running the games for profits. They are limited in financial support and thus the scope of the game will be limited. So it's natural to include the more famous/popular character like Reimu, Marisa, Sakuya, and Cirno first. It's not racist but just realistic.
I dont think it's because Mamizoo is not loli enough. There are really numerous characters that do not show up frequently in the games: Byakuren, Shinmyoumaru, Minamitsu, Aki sisters, Tsukumo sisters, Prismriver sisters... I m sure you will agree that some of them are indeed very "Loli". Hence, I guess a better explanation would be they dont show up not because they are not loli but rather because all game developers are constrained by their time, energy and most importantly, financial resources. They have to be selective.
It is really amazing that some causal works of a Japanese programmer in the last century would developed to such a huge, complex entity of games, MMDs, novels, mangas and such. Everyone who loves touhou has contributed their parts in it. I guess we should not take these for granted. We should support each other instead, and be more appreciative. Hope more people would find Touhou fascinating.

^So, you're loosely implying that, because Mami' is a wholly different character (in terms of char. design), when compared to the [generic] witches, that they overlook her by default -- i.e., on grounds of relative increased programming complexity...? 🤔

Nah, I m not a developer, I do not know whether it will be more difficult or easier to programme mami.

I m just saying if you have resources and some allocated fund to programme a touhou game that can involve, for example, 10 characters only, there is a high chance that you tend to include the more classic characters, and consequently you will miss out many characters. Mamizoo is prob one of them.

And that's natural. If you visit another city, say Singapore, and if you will only have money to spend 3 days there, you also want to visit the more classic/popular/famous sites like Merlion, Sentosa, Singapore River first. You will have to miss out some sites. These places are not necessarily harder to get to, but they are just not classic enough to be included in your travelling plan since you only got 3 days.

I hope that analogy can convey my point.

^ It's difficult to make an excuse for her omission from, say, Skydrift... Thankfully, in Touhou fighting games (the more recent incarnations, at least), she has been present. Too bad she seems restricted that genre alone...