It's a pragmatic one. It has no false negatives, and there's rarely a reason to care about false positives. (Especially not where a stricter regex would (be the only mechanism to) catch it, but a fake-but-valid address wouldn't trivially bypass it.)
The RFC says:
addr-spec = local-part "@" domain ; global address
atom = 1*<any CHAR except specials, SPACE and CTLs>
word = atom / quoted-string
Otherwise we can start saying not just not empty, it also needs no other @s etc. and before you know it we're rivalling the actually correct attempt up-thread. If you just want to catch typos, encourage entering real address (not just a space or full stop to fill mandatory field) then any basic thing is something, a compromise between legibility and how much it'll catch, we know there's a tonne of (mainly adversarial) bad input it'll miss, that's fine.