In Paris they didn't want to build a white elephant swimming venue that will never get filled outside of the Olympics. So instead they converted an indoor stadium, the Paris La Défense Arena that is normally used for rugby and concerts.

You can see in the timelapse video here[1] the pool being built above the surface. They built it to a depth of 2.15m, the minimum required is 2m, but the recommended depth is 3m.

I can only assume that making it deeper would have cost more, and perhaps reduced the sightlines from the stands as the pool would have been higher in the arena?

[1] https://www.thesun.co.uk/sport/29555045/paris-2024-olympics-...