Because when you search through your email, it has to be already decrypted, so why bother?
On the other hand, I cryptographically sign all of my email, so those who use the web of trust have a reasonable assurance it came from me.
On the other hand, I cryptographically sign all of my email, so those who use the web of trust have a reasonable assurance it came from me.
Of course, there's a huge difference between "decrypted on my hard drive" and "decrypted as it passes through N untrusted mail servers".
Heck, it doesn't even have to be decrypted on your hard drive in order to be searchable; just use dm-crypt, or TrueCrypt, or PGP, or any of the myriad other full-disk encryption options.