If you have a computer system that allocates seats, that costs money to buy and maintain. It costs hardware to run it on and then you have to print tickets. You have disputes at the check in desk of who wants to sit where and who booked what kind of seat on the web site. That takes time. Thus Money, Money, Money!
Also, if you have allocated seats, the pax will get mixed up as to where they are to sit and get shirty if they think that someone is in their seat (even if they cannot read their own boarding card. The CC have to sort out these problems and it is quicker (money!) to say "Sit where ever you wish".