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
Since the transformer has tapped holes at its corners which are no
doubt intended to secure it to a chassis, and since sockets are very
rarely mounted with their tops flush with the chassis' top surface
and secured with flathead screws, it's likely that the transformer is
designed to be chassis-mounted with its terminals protruding downward
through a circular cutout in the chassis.
That's further attested to by the nicely bright solder blobs on the
ends of the terminals which, by no stretch of imagination, could be
designed to be socketed.
Blow up the photo and it becomes apparent that there's no index key
for the socket