That's true about fossil fuels. But it seems you're interpreting my comment as if I was defending the use of fossil fuels.

What I'm pointing out here is that the fact that hydroelectric energy production (although very clean) is not really an alternative for many countries as a substitute for fossil fuels. It is not a matter or decision lack of attention or investment. Many developed countries actually have most of their potential capacity installed, yet that accounts for very little of their electric demand. Take Germany as an example:

Germany had a hydropower installed capacity in 2016 of 11,258 MW (...). In the same year, the country generated 21.5 TWh from hydroelectric plants, representing about 3% of the country's total electricity generation.

The hydropower capacity in Germany is considered mature and the potential already almost completely exploited, with limited room for growth. In recent years, growth in capacity has mainly come from repowering of existing plants.

Source: Hydroelectricity in Germany

Of course, there's exceptions (% of total domestic electricity generation): Canada (59.0%), Norway (96%), Paraguay (100%) or Brazil (64.7%).

Actually, from what I can tell, hydro seem to be so convenient (it can be ramped up/down on-demand, used for storage, cheap) that most countries that can afford it tend to maximize their installed capacity to the extend their hydrography allows them to.