Some of them are like the Lake Tahoe Dam and not necessarily as epic as you're imagining.

https://en.wikipedia.org/wiki/Lake_Tahoe_Dam#/media/File:Lak...

That dam is 5.5m according to the linked Wikipedia article. The tens of thousands is a number for those over 15m, so larger than the Lake Tahoe dam. If we are talking all dams, there are around a million worldwide.

Direct link to the study: Perera, D., Smakhtin, V., Williams, S., North, T., Curry, A., 2021. Ageing Water Storage Infrastructure: An Emerging Global Risk.UNU-INWEH Report Series, Issue 11. United Nations University Institute for Water, Environment and Health, Hamilton, Canada. https://inweh.unu.edu/ageing-water-storage-infrastructure-an...

This seems like some stats run on the CIGB/ICOLD's database (world register of dams) https://www.icold-cigb.org/GB/world_register/world_register_...

And notably what they mean by a large dam: A dam with a height of 15 metres or greater from lowest foundation to crest or a dam between 5 metres and 15 metres impounding more than 3 million cubic metres

The knowledge is already there, it's about funding and not overbuilding what you can't maintain. For the same reasons many bridges are reaching old age/end of life.

The US has over 80,000 dams of one type or another.