Old versions of Zecwallet Lite (the QT version) did have full wallet encryption. I came to realize that it was a horrible idea, and removed it.
- if the machine is compromised with malware, the funds are going to be lost anyway, as the malware is likely going to watch you type your wallet password. (keyloggers)
- users regularly forgot their password, and then asked me to help them recover funds.
- a fully encrypted wallet can’t sync (because the wallet doesn’t have viewing keys), and people got upset that their wallet wasn’t keeping up to date.
Overall, in my experience, 10x people lost funds because they forgot their password than they did because of malware.
Like @tromer said, Full wallet encryption is just security theatre and in practice causes a lot of problems.