As was once explained to me by someone with very close ties to iPlayer, it's because these are the requirements that the production companies of the programmes they broadcast insist upon. This extends to their own output due to rights issues, as much of it is licenced out to their commercial arms too.

Usually rights reasons.

A lot of the stuff they show is owned by other companies (films, sports, etc.). They also commission a lot of stuff from outside production companies who want to be able to commercialise it in other ways afterwards (e.g. sell it on to Netflix afterwards, show on repeat channels, etc.).

Other reasons include not owning the rights to things within the show, most notably music.

It's encrypted to prevent other software/hardware from intercepting the signal and recording it. It's DRM.