No, in fact it is simple math

If your servoes draw 2 Amp at the most, your battery is 10 Volt and your BEC delivers 5 Volt to RX/servoes the voltage-drop needed in the BEC is ... 5 Volt. When you starts your calculator you will see that there is a need of getting rid of 10 Watt in the curcuit! (P=U x I) P=5V x 2A giving you 10 Watt... If you put on a battery at 20 Volt the math gives you: P= 15V x 2A which gives 30 Watt! So the main problem is to get rid of heat.

Other factors: the ESC have other jobs to do too, if they should have more sophisticated BECs they would cost more and weigh more.
When it comes to bigger installations the ESCs don`t have BEC at all. Then you use a RX-battery or an UBEC if you prefere. A UBEC with proper capasity have lot of cooling-ribs... the same phenomena happends here of course.

Easy as that...