I wonder why money in escrow account are witheld

So my question is simple whether money that has been deposited to sellers escrow account ( full payment) aftet raised invoice can be accessed by buyer and used for other projectsfor a certain period? Why ? Because project is successfully finished, money are in escrow but not relasead to a buyer for days and sometimes for week and longer. My take is buyer can and does manipulite money for at least certain time between projects or so before he/she must to commit to realease them to seller.