Because we are obsessed with building net new highways. We don't like having to spend money to fix the old.
Also we have competing lobbies in the highway funding space.
The asphalt lobby vs. the concrete lobby.
Asphalt = cheaper, quicker, but only last 10 years
Concrete = expensive, takes longer, but last 40 years now
Since our politicos are so short sighted, they love asphalt. Fits inside the political cycle, easier to fund inside limited budgets and tax revenues.
Kicks long term replacement bucket into a future political cycle.
I have always been a proponent of a scoring system that ranks needs when developing a highway budget. Bridge replacements or replacement of busier, older highways get higher scores than building new ones. If the demand for a new road is warranted, then it will score higher and get funding sooner.