Because the vintage gear you already own is almost certainly better than any decent new gear you can afford!
Nothing you can buy for sensible money now is built like vintage hi-fi equipment, or made in Japan. Nor is much of it very sonically inspiring either, something those of us who don’t make a living selling new hi-fi gear are not afraid to tell you.
I regularly see 40-year-old hi-fi equipment that has come in for its first maintenance. Do you think your BOSE or Sonos ‘thingo’ will be working in 4 years time, let alone 40?! No, and I can guarantee the crappy dock, soundbar or Bluetooth whatever doesn’t sound as good as proper vintage hi-fi gear either.
Experience shows that $500 vintage amplifiers regularly crush $2000 new ones in terms of build and sound quality. If your vintage amplifier only needs some routine maintenance, why wouldn’t you get that done?!