This is actually a good thing, for Mastodon. When you actually use Mastodon, those automatic crossposts are not cool: for example mentions are broken even if the concerned people are also on Mastodon and most of the time the authors of these crossposted messages don't really react on Mastodon so the accounts are effectively bots not marked as such.
I also find that the content is not exactly the same on the two networks, and that some of these crossposted messages are a bit out of place.
(Note that what I'm saying here is about Twitter to Mastodon crossposting, but I really rarely saw it used the other way around.)
Yeah as a mastodon user I actually have the terms 'twitter' and 'RT' filtered, so they don't clog up my timeline. When people want talk about twitter they mostly use the term "birdsite" anyway so that still shows up.
For some it's a way to have a foot both places until they feel secure enough about moving over without losing a lot of connections and engagement.
It'll change. More and more people (and I see it myself too) report far higher engagement even with far fewer followers on Mastodon - our Twitter accounts have acccumulated so much dross and the Twitter algorithm also means a lot of people simply won't see your stuff even if they follow you (as I've run Fedifinder [1] and connected to people I follow on Mastodon, I've many times realised they're someone I haven't seen tweets from in ages, likely because they're people I used to enjoy reading the tweets of but rarely liked or commented on, and so they gradually just fell off my feed).
A lot of the people I follow on Twitter even with quite significant following appears to be tweeting less and less and focusing more and more of their time on Mastodon. Of course this will differ greatly for different interests and demographics.
I also find that the content is not exactly the same on the two networks, and that some of these crossposted messages are a bit out of place.
(Note that what I'm saying here is about Twitter to Mastodon crossposting, but I really rarely saw it used the other way around.)