That's because you buy on a regular basis - it's developing a business relationship. If you had a CTD nearer than 10 miles you'd see how much you could save.
So why shouldn't they be more competitive upfront - because they are retail! If they have a special on a tile say 25% off, you will be lucky to get another 5% for trade! It's the add ons like adhesives, trims etc. that they make the money.
Tile giant have always treated me well. They have a rewards scheme now in place and I'm regularly getting £200 a month back in vouchers to spend in store. Bought my Rubi dc1000 with it!!