Thank you for asking. The answers are actually a very Googleable, you will quickly find many, such as this:
Transformers Are Never Silent (electrical-engineering-portal.com)
PurePower (purepoweraps.com)
Or just imagine this, a transformer has two main parts that would vibrate with an alternating current, at 50 or 60 cycles per second. Namely the core and the windings, the cores are typically made of laminated sheets. Some are wound tighter than others but they will all vibrate because of the applied AC. Anything that vibrates will make noise. Normally this is not really an issue because no one listen to their amp's transformers and they don't listen to music from a few inches away from the amps either.
So those who complained about highly audible hum would likely have some sort of power quality issues that are either there all the time or only when certain devices in the house is running and contaminating the line quality.
DC offset is one most often cited as the culprit for louder than normal hum.
Mains DC and Transformers (sound-au.com)
DC Blocking for Powering Audio (kccscientific.com)
Not all transformer hum the same, some are more immune to power line quality than others. In my house, I can plug all my gear into the same outlet, say one at a time, and some, such as the Denon or Marantz AVRs, my NAD integrated amp and others would not hum audibly from more than an inch away, but the Halo A21, the 4B SST and some of my subwoofers would hum loud enough to bother me from several inches away, in fact two of them are audible from several feet away. So yes, you may be lucky that you have amps that are more immune to poor power quality, or your power line quality happens to be excellent.