Material strength vs the forces applied. Zippers are perhaps rarely _just_ holding two same tension fabrics together. Often they are also supporting forces which press against the materials laterally, such as with bags.

So you have some small pieces of plastic, namely the little tooths of zipper material, which must interleave and hold against forces trying to pull them apart.

But even more than the static force against them is the force preventing the teeth from "sealing". Each zipper tooth has a somewhat conical outward protuding piece and corresponding tail indention to host the following piece. Forcing these cones-within-divots to interleave properly and stay in place is not easy. The materials involved must be strong but also a little pliable.

Zipper tech is likely much more complex than would appear from the user perspective.

Edit: also, in contrast, velcro doesn't have to connect perfectly as a zipper must. It can connect with a subset of overlap and still provide closure (in the literal sense). As long as what remains connected stays closed, the velcro is effective. But a zipper where a few teeth do not connect tends to break surrounding connections, leaving a large gap.