Because that gives you some degree of design. There's an enormous difference between "looks the same" and "looks like 1996". The average end-user has basic expectations and will very likely leave if those expectations aren't met, at least until they've already bought in.
For certain kinds of site this look could be a positive - thinking of Pirate Bay, 4chan etc. Not the sort of things that get VC funding granted, but where a non-corporate, amateurish design gains you street cred.
Personally, I trust tarsnap way more than I would if it were bootstrap-looking.
I am more likely to leave if the site looks overdesigned, honestly, because that is a signal that this company is focused more on the "acquiring customers and extracting attention" side of the business than on the "providing a useful service" part I'm actually looking for. If the site looks plain and practical, it might turn out to be useful; if the site looks gorgeous and cutting-edge, the company is probably just going to piss me off.
If the site is plain, it looks hand-crafted; like somebody who cares about it made it, and they're too busy making awesome content to fuss about the paint job.
If the site is "designed", it looks like someone just hired a designer to sell me something.