Because git combined with Dropbox and the likes can be slow, lead to broken repositories, mad sync conflicts and so on? At least that is what I've witnessed with colleagues. This was years ago though, and I assume you dont have these issues or maybe a different workflow, so this is more like a word of warning for others: test this properly first before just diving into it.
Anyway, YMMV.
Are you sure those people were using their own personal dropbox folders? If they were sharing with each other that's a different thing entirely and requires a lot more care to avoid problems.
Yes, one single account.
while it's syncing and then start editing the same things on a different computer
IIRC this was one of the issues. Not necessarily because of shutting down, but just when using multiple computers one after the other and Dropbox not having synced in between. Not that niche depending on line of work, think lab-like setups where one thing gets deployed to multiple machines and deployment then gets tested. In one company the 'oh but you have to wait for dropbox to sync before doing that' and 'how?' responded by 'urm, yeah, check the timestamp on all files in that directory' was a rather common thing, often leading to stupid issues. Put git in the picture there and things won't get any better.
And even then it's just something like having disagreement about an index file. Easy to resolve.
Though outside of garbage collecting, git doesn't have many files that actually get changed. Mainly the ref pointers which are easy to resolve and the logs which aren't very important.
A git repo in Dropbox can appear to work okay with a single contributor where that contributor is only working on one computer at a time. But as soon as you add a second contributor, you very much should take Dropbox out of the process.