Because:

a) speaking "black shoes" into your phone is a better experience than scrolling down an arbitrarily long list of categories.

b) Is shoes a category? Is it clothing? Is it boots? Oh I need an ontology now? Will users find it intuitive to have to drill down on it???

c) Wait, the item comes in with an automatic description from a thousand different vendors. How do I decide what part of it maps to my "color" field? What if the color isn't in a separate field? How do I make sure that this item isn't dead in my inventory? How do I make sure that this popular item is retrievable for me too and not just my competitors that have a better search??? In this case just searching for "black" anywhere in the fields of the product, weighted for significance, gets me valid results without communicating to the user that they filtered on a color.

What do I do if I want burgundy shoes? Do I decide if I think it's red or purple or blue? Or do I go further and think what the person listing it thought it was? Do I hope the decision went the same way each time, and all shoes have their colour listed?

I don't do any of this. I filter by shoes and look with my eyes. Retail search is a poor example because existing retail tagging is poor.

Lucky you. I search eshops with

  black shoes site:https://shittyeshop.com

on Google because of abysmal search on 99% of sites.