Question Theme Engine Limitations

Hi,
i was wondering if anyone of you can tell me, why the theme engine can theme close to everything but the lockscreen, power menu and some other small things like volume controls?
Do you know , if someone is trying to get rid of these limitations?

thanks for reading
[BootAnimation][CM11] CyanogenMod Inc. Logo [new Engine]
[ROM][4.4.2] Nexus 7 3G (2012) Omni-Rom Unofficial [with Multi-Window]
[Themed][App]Sony Ericsson LiveView AOSP/Vanilla[Yes, watch's screen itself!]
[Themed][App]Sony Ericsson LiveView Black`n`White[Yes, watch's screen itself!]
HTC Dream/T-Mobile G1 >Cyanogenmod 7.2<
Samsung Galaxy S2 I9100 >Cyanogenmod 11 nightly<
Sony Ericsson Original LiveView >OpenLiveView<
iPod Touch 4G 32gb >iOS 6/Jailbroken<
Asus Nexus7 3G >Paranoid Android 4.5 Beta<