....why is cell recording allowed,...
AFAIK call recording is not a feature of stock Android, though certain OEMs and custom ROMs enable it or can be done using apps. I guess legal restrictions is the reason for not enabling this feature, as call recording laws widely vary
VOIP recording is quite different from voice recording
Firstly, it is possible:
App recording VOIP calls is possible, see Call Recorder - IntCall for instance. There may be others too
Non app based services that record VOIP calls seem to be more - Dingtone, for example
What is common for VOIP recording and different from voice recording is that VOIP recording is done when it is routed through the servers provided by the app (see description in both links above). Ordinary voice recording is not bound by such limitations and can be natively record if enabled on device
Call Recording Software Wiki
is usually restricted to streaming media recorders or software developed by the softphone or IP PBX creator. There are also solutions which use packet capture technology to passively record VoIP phone calls on the LAN.
Since VOIP is using data packets to transmit voice, these packets need to be actively or passively sniffed ( see wiki above for details) and this requires dedicated setup unlike voice recording ( which can be done natively on phone (if enabled) or using apps ). Hence, VOIP recording is not simple to implement on device like voice recording- a technology constraint