There is a lot of judging of ideas here as though there was an answer that
was knowable by us. There are possibilities and time, chance, and the
environs have given us a result. Why (interestingly enough) is much harder
to decide than "why not". A Venus Flytrap uses food and photosynthesis (probably)
because poor soil yielded to little nutrients. "Green People" (being people) might
have selected mobility -- for a number of reasons.
Photosynthesis does stink in terms of making energy in some respects -- maybe we should
do a quick energy budget -- I use 2000 Kcal/day -- how much green surface at 45N in mid-winter
do I need to generate that sort of energy flow. I suspect, I'd have to be as big as a tree.
It does have the distinct advantage of being one of a very few ways to make energy
from inorganic sources -- hence its the begining of the food chain