Channel subscriptions offer a significantly more reliable revenue source to a broadcaster as bit usage can fluctuate heavily and can’t be relied on month to month. Also the benefits of subscribers such as emote unlocks can be a significant added value, and some of partners may have more benedicial revenue shares with Twitch than the default 50/50 on subs.
Should a user wish to just tip a broadcaster they can also do that through cheering what existing bit balance they have, or through a 3rd party payment processor, both of which offer significantly better revenue for the broadcaster than Extensions.
One thing to note, I’m juay talking about the type of extensions that are generally available for developers to make. I can’t speak to the revenue of Extensions that are part of the commerce beta and can sell merch through the Extension as only a select few Extensions are granted that and as far as I’m aware they’re no longer accepting new Extensions into that program.