Offers with an acceptance deadline simplify life for the employer, so they do that. Refusing to accept the deadlines or reneging on acceptances simplifies life for the applicants, so they do that. Employers can respond by rescinding offers when someone better comes along, which applicants can counter by accepting and holding as many offers as possible. Employers then realize they must make multiple offers for each position with the intention of rescinding those made to applicants worse than the best one which accepted. Applicants likewise renege on every acceptance except the one from their most favored employer amongst the employers that extended them offers. The time scales on which this all happens compress until it is essentially one big chaotic race condition. When everyone is predictably acting in support of their own interests, the outcome is also somewhat predictable.
It's interesting to observe the tech intern labor market retrace the path that other candidate / organization matching processes, ranging from sorority rush to medical residency applications, have gone down. The amusing bit is that the apparent endgame, where both sides submit ranklists to a central clearinghouse that uses some form of the stable marriage algorithm, sounds like something a tech company came up with.