There is an easy reason.
Imagine you have 3,5M and you see a guard for 3,5M(and maybe the guard is worth around 2,8M-3,5M) most of the times you will buy the player bidding for 3,5M instead of waiting some days to see if the manager of the player list him for less.
That's why managers try to put high prices and if they don't sell it they put it again by the same price or maybe 50k lower, they just wait a manager to bid on his player.
Most of the managers that i've seen doing this strategy have great benefits from the transfers and if you do many transfers during the season you can imagine the profits they have...