Time-limited pickups

It's common for pickups to disappear from the game world a short while after spawning. The period varies, but is typically on the order of a few seconds.

This mechanic originated largely for technical reasons. With the limited memory capacity and processing power of early computers, it was not practical to keep large numbers of objects in the game world indefinitely.

This usually isn't a problem on today's more powerful hardware, but the mechanic endures because it adds a tactical element to gameplay - players must keep track of which pickups will soon disappear, and weigh the risk of acting to grab the pickup against the reward. (In games which involve a lot of pickups, it can also help to reduce visual clutter.)

It's common to have a visual indication that a pickup is about to disappear - usually by blinking a few times.