Besides being intermittent, solar and wind are not really dispatchable, that is, the grid operator doesn't have many levers to control the power output of a plan, and thus this imposes more stress on the other dispatchable power sources.
Some of those backup sources are not very flexible and take a long time to turn on and off, like coal based, and a lot of nuclear plants. Others, can be brought up online, ramped up and down faster, like gas turbines and hydro.
But other than gas turbine, most other firm sources economics are based on a predictable demand and a minimum duty cycle. A nuclear plant is very capital expensive, have an excellent capacity factor, but, it can't pay itself and its investor if it is not going to be run most of the time.
Base load is cheaper, because you dilute fixed costs, peak load is more expensive, because you sell less units to dilute your fixed costs.
Despite whatever the renewable lobby says, experience has shown over and over, that after a certain proportion of intermittent generation in a grid, large frequency excursions, deteriorated economics and frequent load shedding events are rather the norm than the exception.
AC grids are stupidly complex beasts. Most politicians, journalists and investors that drive our current discourse on the grid don't have even the most basic pre-requirements to understand it.