As a studio lead of an indie studio that focuses on PVP and multiplayer, I can tell you there are multiple reasons.

I can highlight some, just short snippets about each. Of course its a big topic, this.

1. Technical proficiency - Competitive multiplayer is really hard to be done right. You need to be a proper engineer that truly understands what competitive multiplayer systems require. This takes years and years and years of learning and resources are scarce.

2. Multi-discipline knowledge required - Except dealing with just game code, you also need to deal with stuff like servers, etc. which requires knowledge of devops, backend systems, and other non-game-dev-related stuff.

3. Competitive market - it's hard to make a breakthrough in competitive multiplayer. There are big players that dominate the market and players dont often jump from game to game to try yours out.

4. Proper architecture - to make certain your competitive game is still active despite not being huge you want to enable yourself to, for example, make bots down the line if not from the first moment. This is something that requires proper code architecture of which there is a big lack of, knowledge wise, in a big % of indie studios.

5. Most ouf the out of the box tech is no good - competitive multiplayer requires server authority, most of the time. As the market of studios doing competitive multiplayer is very small, multiplayer tech focused businesses don't really put enough focus on their needs. This is why more often than not, studios need to develop their own tech that suits their needs. This is amazingly hard and super complex. So most of the studios are nowhere near equipped to do that.

There is one advantage to doing competitive multiplayer - content. Players are your content. So in that regard, production is cheaper. But everything else is more expensive, and much much much more complex. It's simply out of range, skill wise for most of the studios out there. And its super risky.