they don't really last as long as the packaging says

that would be you switching on and off. The LED bit might last 20,000 hours (MTBF) in constant use but the circuitry works at probably 1 Mhz with coils and magnetics and silicon so your switching on/off still affects the life of what I assume would be the FET and circuitry generally.

What's causing my LED light bulbs to flicker?

yer problemo is the nature of the AC voltage and the way the LED circuitry works. The dimmer will merely switch on each half sinusoid at (in the US) 60 Hz. The LED drive circuitry is (presumably) designed to smooth out the rectified AC and expects it to be a certain shape (sinusoid on each half cycle ie all positive). As the dimmer starts to not switch on each half cycle for a portion of the half cycle, then on for the rest, there is less time for the circuitry to accumulate a store of energy to fill in the gaps. That would cause flicker.

Then you have the problem that the dimmer is a high speed switch, with harmonics extending up into the Mhz. With the circuitry running at that frequency they can interact. ie a Beat frequency, analogous to the twelve string guitar or mandolin. AFAIK you tune the string pairs to beat at a pleasant rate (frequency).
The solution would be to buy the LED and dimmer together as a combined unit, or at least from the same manufacturer.

For a beat frequency to appear you need non-linearity. Circuitry has plenty of that, so does the wood and construction of the musical instrument.

FWIW my strip light dims when there is no body to trigger the PIR, and given the uniform light that results I would guess the dimming mechanism is as per separate dimmer, rather than altering the number of LED dice powered. It powers down to 1/10 which I use to see in the back yard on return, and put the key in the lock!