Generally speaking, only a tiny fraction of extensions with in-extension transactions generate significant profit. Many make enough to cover operational costs but not huge amounts beyond that. There are of course some exceptions, but they are a rarity. There are more cost effective ways for viewers to support the streamer (which will also be more worthwhile for the broadcaster) than extension transactions.
Broadcasters who want something designed specifically for their channel, yes. Just like more professional streamers go to artists and musicians for channel art and music to help their brand, some also commission developers to produce extensions, bots, etc… It’s not common, but it’s not unheard of either.
There should be more options, both for revenue share so that I can set my in-extension transaction to 100% to the broadcaster, but also other options such as ads, and subscription benefits, in some form. If you want suggestions, go look at UserVoice, that’s where feature requests are appropriate.