Because a lots of people on the spectrum learned (or forced to learn) how to hide the obvious behaviours and you just can't tell about them.

After I was diagnosed and I invested some time into studying about autism, I realised that there is handful of colleges, who are most probably on the spectrum, but they have a really systematic approach on hiding shortcomings and using autistic traits to their advantage. Also I'm pretty sure I have collegues, who hide it so well, that nobody notices it at work.

Not all autists have the same collection or severity of symptoms, though, so it's not always obvious.