Clearly not the case, as you’ve demonstrated in the OP. Sometimes you’re dealing with a niche product, where someone isn’t willing to produce/sell for peanuts because the market is just too small.
Case in point: I have a product that I sell on the internet. It’s a tool that works together with another commercially available tool. Producing in quantity, it costs me about $3 (and three minutes) to make it in my basement, package it, and ship it; I sell it for $30. To my knowledge, nobody else makes anything like it, which is why I can charge that much. The market is pretty small though, just a few hundred units per year, so it’s not surprising that someone else hasn’t made arrangements to have pallet-loads of them molded in China and shipped here.
Same might be true of your cell phone. I wonder how many copies of that particular cell phone Verizon sold? After that, I wonder how many of those particular cell phone owners were interested in a screen protector? If there’s only a market for a couple thousand screen protectors, few manufacturers are going to be excited about tooling up for that.
The economics look different if you’re dealing with a LOT of units. Take the iPhone, for example, with 75M units sold. Now you’ve got a big market, with plenty of room for third-party accessory manufacturers, and indeed you do see all kinds of variety in price, quality, and name brand.