Why can't encapsulation ppp be enabled on Ethernet?

Question

Cisco

Getting ready for CCNA. I perform test tasks on cisco packet tracer. I made a connection diagram, but it turned out that encapsulation ppp cannot be enabled on the Ethernet interface. It is possible only on Serial.
The question is why?
Another question: how is one different from the other?

Answer 1

for Ethernet, the Point-to-Point Protocol over Ethernet protocol is used - PPPoE
Ethernet interface - is meant for connecting via LAN, and Serial - for WAN