Question

why do we call plants univarsal food providers?

Answer 1

because they make food for the world by storing the extra food firmed during the process of photosynthesis

Answer 2

because only the plants can do the process of photosynthesis.
And prepare there food with help of sunlight
water
carondioxide
in the presence of chlorophyll.
Which causes they make there food. And the all the humanbeings and some microorganism and animals get there food by the plants directly and indirectly.
hope it helps you.