Just because the trade deadline has passed doesn't mean that teams still can't improve. It's just a bit more complicated now. After the deadline, players must go through the often-confusing waiver process to be moved.
During the waiver period, there are controls on player movement. The waiver process was put in place so teams at the top of the division cannot unilaterally load their clubs for the stretch run. It is a system which favors the teams behind in the standings. Teams with lesser records have the first opportunity to improve themselves and can also block better teams from making deals.
What complicates these trades is the timing necessary to move multiple players through waivers and between teams. It can be done. My first trade as general manager of the Mets was a six-player waiver deal which sent Lance Johnson, Mark Clark and Manny Alexander to the Cubs for Mel Rojas, Turk Wendell and Brian McRae.