Question

why animal cells are flexible and plant cell are rigid

Answer 1

cellulose present in plant call wall makes it rigid whereas animal call is not made of cellulose.

Answer 2

As I would like to tell that plants and animals are multicellular
The plant cells have cell wall but the animal cell does not have any cell wall it only has plasma membrane as it's protective layer which thin and flexible while cell wall of plant cell is thick and rigid which gives it a shape mainly made of cellulose and pectins