Because there's no other material with the same properties, at least one that can be made cheap enough and won't fall apart by itself. It's just like oil and gas, we use it because we don't know anything better.
The only way to replace plastic is work hard on making different materials that beat it in every way and scale them up with massive state investment. Then you're fighting multinational companies, neoliberal economists, anti tech green activists, skeptical scientists and corrupt politicians all at the same time.
My personal view is on minimisation, not elimination. Some things practically must* be plastic. Where they don't (like microwave/storage containers, pre-wrapped vs loose item, etc I opt for the plastic-free version.
*economically/practically, not absolutely
I completely agree with you. We use plastic in scenarios where it's just not needed. Bags, packaging for veg and fruit, packaging toys, some toys in general, and so on.
Not everything needs replacing, a significant amount would be fine just by not using. Produce does not need plastic introduced and would be a significant change. Cold drinks in coffee shops do not need plastic cups. Toys don't need to be packaged in plastic and so on.
We have some alternatives that work, and some scenarios where we still need to use it, but we can reduce the use significantly. Like Oil and Gas we absolutely do know better in many scenarios and just choose not to.
Sounds so much like the arguments for asbestos.