Probably cost and quality?
The 2nd hand value of a bike is about 50% of whatever it would be new. So for most mountain bikes outside STW that’s probably £500-£750.
So the sellers expect that, or they could put in on ebay for 7 days and get it.
The buyers expect that or they could buy one off ebay for that.
And as it’s a shop they also expect the shocks serviced (£200) and, new drivetrain bits and pads (£100), and a good service (£100 parts and labor).
So now your £1000 RRP Trek is £500, plus £400 in stuff, and Pauls Cycles are knocking them out the door for £650.
Some LBS sell 2nd hand stuff, but it’s mostly higher end stuff (£2k second hand Yeti’s and similar) taken in PX so the profit was made on the new bike and there’s only really the cost of a service. More common in Road Bikes too as kit has an easier life and wear and tear is more evident.