why do most thunderstorms have flat tops?

Answer 1

The cloud would be able to rise higher were it not for the fact that the troposphere ends at a certain height in the atmosphere (between 10 to 20 km altitude, at the equator it's 20 km near the poles it's 10 km).

What happens is that the temperature gradient in the atmosphere suddenly decreases, and then the temperature will start to rise as a function of height (temperature inversion).

Now, clouds cannot rise when there is a temperature inversion, so what happens is that the cloud will expand in the horizintal direction. This leads to the flat top.

Above this point the stratosphere starts (the region between thepoint where the temperature gradient decreases and the point where the stratosphere starts is the tropopause).

The temperature inversion in the stratosphere is caused by ultraviolet rays that are absorbed in the stratosphere (which also leads to the formation of ozone). The energy that is dumped in the stratosphere causes it to heat up, and more so at higher altitudes.