Because the plug may have been butchered by previous Muppet.
Because they run the risk of ballsing it up and rounding the plug.
Because sump plugs and washers cost money.
In addition if they can get to the filter without lifting the car.
Because it takes time to lift the car to a height so they can drain it and lower it after.
Because a lift costs money. The per working hour cost of finance, insurance and maintenance added to the oil change would make them uncompetitive. They can put more profitable brake jobs on the lift and do the oil change on the floor.
Use of the lift requires extra training of staff for H&S at work, usually higher skill level and higher pay (the guy that does brakes). They can use a lower paid Muppet for on the floor oil change.
Use of lift runs the risk of damage to lift points (not that anyone notices).