Mass car transit wasn't their intended purpose. They were meant to be a small go-between for local use, not the primary method of transit. Which was part of the problem when people wanted to Westernize the country without access to super-exploitation.

The roads intended purpose was for public transport, business use vehicles, emergency vehicles and bicycles. They simply were never designed for the exponential increase in wear and tear. Like no-one involved in planning expected car ownership going from 1% of households to like 40% within a decade.

They would have built them sturdier to begin with if they expected heavy traffic regularly. It happened to China too, when we shifted from bicycles to personal cars, although it's getting better again with mass transit infrastructure being the focus of this century so far.

Except they were used for their purpose. The fact that individual households did not have their personal cars doesn't mean that they were not used. In case this concept is alien to you, the USSR had public transport.