[fragmede]

Packet size is dependent on the MTU, so practically speaking you're trying to put something 1460 bytes into a 1460 byte container and the only way for that to fit is the split the packet or tell the packet generator to make smaller packets. both of which are reasonable options but they're not the most efficient, leading to slower connections when tunneling one inside the other. It's less of a deal theses days, but that's the why of it.

[g1sm]

Well, sure, but then any kind of encapsulation is less than ideal. However, here the context is VPNs and this means there’s always going to be some sort of encapsulation. And if the choices are between encapsulating something in TCP and encapsulating something in UDP, the latter should always be chosen.