The real and honest answer is because repairs are done cheaply. Asphalt taken up and replaced with 4" of new asphalt in this climate guarantees a relatively short lifespan. Thus, they really can't keep up with the repairs.

If you want to really improve things, you need to replace the road with 6-8" of asphalt. But you're significantly increasing the cost of road repairs and given that most of the budgets for this stuff is planned years and advance, those same budgets are eaten up by inflation (rising prices of equipment, consumables, etc.) and those accounts are dipped into for other things.

Want to truly fix it, increase taxes, increase thickness (THE THICKKKKNEESSSS!!), and get rid of the whole "Only police can flag wave" BS argument. Until then, put monies aside for new struts.