Agreed, in general, but not always.
If it is short trip then you're correct, the weight and volume of solar is better replaced with an added or bigger battery.
On longer trips it begins to shift to solar.
A 21W solar panel in good conditions make about 50Wh per day of energy. That is about the same as a 13000mAh battery. For most a 13000mAh battery is about 2-5 days of energy needs. So solar is making 2-5 days of energy, so once you cross over into >2-5 days then it becomes useful. Accept that is rarer need.
Also in home power outage emergency situations where weight is not the factor, solar can be cheaper than alternatives.
In my home I have two 20W solar panels, that is enough to keep a family of 4 with torches working through a long power outage "indefinately" (*)
(*) your cellphone signal disappears anyway as the cell towers run out of power from their UPS so you're mostly needing to last a few days for local issues like storms with lots of downed power and the cellular service is fixed first and home power fixed 2nd.