Why do air pumps get weaker over time?

Most air pumps operate by vibrating a rubber diaphragm. Combine that with check valves and you get air moving in one direction. Over time the rubber degrades - even cracks - leading to lower volume (or no volume if the crack is large enough). You can buy replacement diaphragms for many pumps, but retail cost plus shipping (not stocked locally for me) usually makes a new pump a better deal. The check valves also could become an issue - any debris could keep them from operating correctly.

Kevin

If you can get to your diaphragm a fine coat of silicone lube can help keep it from drying out.

Sometime moving part of pump is not in right position any more due to vibrations so output drops.
Also, good idea is to connect multiple outputs of the pump into one to have bigger output. People have pumps with 2 outputs but use only one

Usually it's dirt accumulating around the flapper valves. Taking the pump apart and cleaning the silicone valves and the housing with a q-tip dipped in rubbing alcohol will make quite a difference IME.
As far as quality goes, you are better off buying a good pump. I use the Whisper AP 300 (aka Tetra DW) air pumps. The AP 150 is similar but only has one outlet, the 300 has 2 outlets. I tried the Whisper 100 (blue dome) dual outlet, but the diaphragm on one side will only work for a couple weeks before rupturing. I replaced it twice, then gave up and bought a better pump. The AP 300 will run 2-3 tanks from one of the 2 outlets (heavy airflow, not just a weak stream of bubbles) well enough to circulate the water in a tank with a single airstone. Also works very well with hydrosponge filters. I have 3 of the AP 300's running 3 75's, 2 55's, 1 45, 1 40, 1 38, and 2 29's.