Because generally a product you wish to sell to the public, and it's cost to make, or perhaps buy from the maker has very little connection.
Your aim as a seller is to maximise profits.
You wish to sell an item right on the sweet spot of what the public are prepared to pay for it.
You price it too low, you sell a lot, but people would have paid more, so you lose potential profit.
You price it too high, you don't sell as many as people are put off, so you lose potential profit.
You price it just on the cusp, which is what you want to do, and you get the perfect balance of profits per item and number sold.
Hard to get exactly right, but it's what to aim for.
If I can source a phone from china, and it costs me, £100, I slap on Import Tax/Shipping, VAT and let's say it's £150, I slap on some clear profit for me of £50 per item, and let's say I can then try for a £199 tempting price.
But hang on. I look around the UK Phone marketplace and see the typical price for anything like this, and what the UK public are happily paying is nearer the £299 - £349 range.
So, my item is under-priced for the UK Consumer/Market, I'm missing out on profit, so I can slap on another £100 and get to the price I see is selling well for the specs of this level of product.
Lot's of made up numbers here
Perhaps 3 don't do that, and they will take the factory price they can buy them for in bulk, add on shipping and tax's, plus just a few % profit, and sell the phone at an amazing UK price to us over here.
I'd love it. And shake up the competition