it becomes more problematic at the center.

This is precisely what I’m trying to understand: what gets more problematic? The driving? The civil engineering? What is the exact complexity that a center drain would introduce?

some one has to go unclog it.

Conventional drains along the curb also need to be unclogged manually, except that the public works dept needs to get people to move their parked cars, which can also hide the problem from being easily noticed in the first place.

either have to shut down both directions or be at risk of being hit from both directions.

From seeing how my town accesses manholes located in the middle of a two-way road, they arrange two heavy trucks in a row, one before and one after the manhole. On a multilane arterial, this is a minor traffic disruption of one lane. On a quiet residential street, people just go around slowly.

with the way roads are constructed, it would be a lot more expensive to design the sewers to either tolerate the loads (imagine a big heavy truck,)

I believe that roads are infact designed with the sewer and storm drain pipes directly below the traffic lanes. See my other comment. A cursory review of my town’s planning documents for a new road extension shows a cross section that has all longitudinal piping underneath the lanes, so that the manholes aren’t installed under sidewalks or the curb. I am open to seeing plans for other jurisdictions that build their pipes differently.