Transformers are generally soldered in, and often last for the life of an
amplifier. Tubes are socketed because they are less reliable than a
socket, and often must be replaced during the life of an amplifier.
Sockets themselves become a potential failure point, and they cost
money. So why add expense and diminish reliability?
Transformers are often chassis-mounted, because they're heavy.
So, to connect the transformer to a printed wiring board, it's
VERY common for a PWB to have a socket for the transformer
connection. The flexible leadwires from the transformer
are soldered to the solid (inflexible) windings, though.
Transformers (and other inductors) get a bad reputation partly
because it's difficult to put them through 'standard' pick/place/solder
operations.