The road network expansions and maintenance required to support the suburbs does. Ottawa has this exact problem. They are widening inner-suburb roads to satisfy demand created by outer-suburb development. A double-whammy.

Those suburbs outside the city limits still need money. They get it from state and federal funds which were mostly collected from people in the city limits. For example the Highway Trust Fund as one of many examples. If you check the per capita spending, it's higher for suburbanites than urbanites but the urbanites are putting in more money.

DC is an exception, most American cities have large swaths of suburbia within the city limits and even larger ones within the same state.

“Seem to be” being the key words.

There’s been somewhat of a continuous cycle of movement with the upper class of America that goes something like this:

1. Move in to the latest newly constructed suburb with low taxes and fresh housing stock

2. Infrastructure ages, tax rates increase, housing standards evolve, the upper class moves to the next new suburb built on former farm land

3. The inner suburb declines, becomes low-income, companies leave, and infrastructure crumbles. Municipal debt piles up, sometimes the city goes bankrupt.

This is especially obvious in cities where regional growth didn’t continue upward forever.

I recommend reading up on Strong Towns’ growth ponzi scheme content.