Warning: Energy geek-speak ahead.
To maximize the Power (P) from an electric generation plant, Voltage (E) and Current (I) must be "in phase". That is, the sinusoidal peaks and valleys must line up. They do by design (within certain limitations to account for reactive load [VARS]). But when operating more than one generating unit on the same grid, the units must both be "in phase" with each other. That is a slightly trickier proposition, but not difficult. With one unit already locked on the grid, the oncoming unit's speed is varied so it is slightly "faster" than the grid. The grid normally runs at 60 hertz, so the incoming might be running at 60.02 hertz. When the phases line up, the unit is locked into the grid. By locking it in slightly faster, it picks up some of the electrical load and maintains stability of the grid (in very simple terms).
Wind power has no such ability. The wind is the wind, and although there is some ability to vary windmill speed within a local group, but varying windmill speed to parallel units between an area with a 20 mph wind speed and a 10 mph wind speed can't be easy. An the variation in wind speeds makes it that much more difficult. Easier to dump the energy from the windmill to a battery and parallel a motor/generator to the grid. Maybe it's done that way, I honestly don't know. But it is inefficient and brings a higher capital cost per installed MW.
PV Solar MUST be done using some sort of storage device and a DC/AC conversion arrangement. PV will only output DC.
Again there is a problem with wind not blowing and sun not shining (or shining at a very reduced rate). Spinning reserve must be maintained in order to pick up when either of the "green" sources falter. Having run a plant providing that spinning reserve, I assure you it can get exciting in a hurry. As an example, a large 1000MW nuclear unit tripped off-line and took another 1000 MW unit with it. My available 150 MW units along with several others on the grid attempted to pick up that load. It was a**holes and elbows in the control room as grid frequency swung wildly, nearly hitting the point of cascading failures all the way down the grid. At the same time, we started to fire up one "hot standby" unit and two "cold-iron" units. It seemed like an eternity before the grid stabilized. This is going to be standard operating procedure when wind and PV solar become the 20% the administration wants. It will be in.....sane.