Why is it necessary to reduce the number of chromosomes in gametes, but not in other cells?

because you don't want to double the amount of chromosomes every generation

"That doesn't happen that way," say all plants.

You should go with someone else's answer. Someone else will explain about how meiosis and then fertilization result in having the same chromosome number in subsequent generations.

recombination to a 2N zygote can only occur with 2 1N gamete cells all other cells are 2N.