IMO, that tool probably doesn’t exists mainly because it’d be a thankless pain in the ass.
In general, unless you’re self-hosting Mastodon, you probably mean “my Mastodon account on someone’s server”. And that “someone’s server” probably has some moderation rules which differ significantly from Twitter. So, re-posting a bunch of tweets (i.e. “transferring”) to someone’s server can get hairy. Maybe some or all of your past stuff isn’t copacetic with the new server - but who wants to re-litigate a sudden influx of old posts?
So, IMO, whoever writes (or uses) a tool to copy over a bunch of Twitter posts to any arbitrary Mastodon server is probably going to get yelled at a lot.