Yeah... and let's say pressing the button puts the transaction on hold until a GA investigates (because GA's aren't omnipotent, I can imagine enforcing such a button would triple or even quadruple their workload, so this delay could take a while).
Anyone in my league, or various other FLs or cups, could browse the domestic transfer list and stop me from making perfectly fine transfer bids on players I want/need to purchase.
Which would make setting up tactics for an important match quite a headache if I wanted to make a quick purchase.
And as navai said, one person could freeze all the transfers with minimal effort (some people just want to watch the world burn) which also effects the sellers, it could place them in massive financial problems if they needed to sell a player to avoid bankruptcy.