Because labour is crazy and I don't feel comfortable doing this myself. I've recently had a basic AC unit installed at home, it was £500 for the hardware and then another £1000 on top of that for the installation. I imagine that exchange fan would be at least £1000 installed and I just don't have a grand laying around spare to do this.
Yeah, AC units do require a lot of specialized labor to install (in some jurisdictions, it actually may even be mandated by law)... but a simple fan? That's half an hour for the drill, half an hour for the electrician and that's it.
