You have to distinguish between an airline operated lounge (i.e. any OneWorld airline operated lounge) and a third party contract lounge.

Any airline operated lounge would not impose a time limit. They are aware of connecting flight times. AA do sell day passes so no time limits are involved. These lounges are not normally a profit centre for the business.

A third party contract lounge makes money from charging - they charge both passengers directly as well as airlines who use them. These lounges are there to make money. The reason they limit use of the lounge is to minimise consumption of food and drink and it also opens the opportunity to collect two sets of charges for an long stay; i.e. you just pay again to extend your stay in the lounge.