It's not that they "don't" want to do that - in fact they'd very much like to. It's that they can't do it. Their agreements with listing services prohibit this. And if they don't get listed then they simply won't get the same rate of bookings.
Generally they are paying around 20% of the booking fee to the listing service. So in theory the hotel would be able to charge 15% less on their own site and make a larger profit. But if they did they would no longer get listed on the popular sites where people go to easily price compare and then book.
The loophole in those agreements is that the best price that listing services show is the best "public" price. Larger hotel chains are working around that by offering lower prices to their loyalty club members.
If the hotel offered the lesser rate themselves, they would effectively be pricing their "free" amenities, and they don't want to do that.