There are a lot of reasons for that. like - no revenue - made by beginners with no scalability in mind so it simply cant handle the traffic if a bot gets popular - lose of interest in developing if it doesn't "blow up" soon - every good idea is instantly copied especially if the code is public there will be clones all over the place. - no official way to promote your bot. Most large groups will directly ban you if you tell em about the bot you made. Some even have bots to auto-ban if you name another bot. Everything is considered advertising/spam very toxic behavior in a lot groups. As with everything free on the internet there are the 0.1% who create and all other consume and give back nothing.

Source: I'm on Telegram since nearly day one. I run roughly a dozen completely free to use bots since many years. Some of them with thousands of daily users.