Because I don't expect a random thief to go to the lengths of identifying OS level vulnerabilities to bypass the login prompt in order to decrypt the disk contents. The potential gains for them are marginal compared to the time and effort spent there, not to mention technical expertise needed. I expect them to steal it, and if it's encrypted, just sell it for parts, or wipe it and sell it anew.

Entering a password can be a lot of work if you use a strong password (and if you don't, why bother with a password?). Typos can take a toll too because of all the delays included.