At a guess because they have no experience or knowledge on dimensioning something made out of concrete.

You can land an A380 on a runway 17" thick...

This was definitely strong enough, just not ready to withstand cavitation and given that bronze ship screws suffer from it it is hard to see what kind of liner would have solved this problem entirely.

Solving it completely may ask for a totally different design, likely not a different material or thicker concrete.