Parts warehouses tend to want to ship things via express services such as DHL, and even the ones with cheapest shipping still have higher shipping costs than "random Chinese seller in Amazon, eBay or AliExpress".

I'm in Europe, so Mouser shipping for here is very expensive (around 30 euros via DHL if I remember it correctly) so I use an European parts warehouse - TME, which is in Poland - but that's still 8 euros for the cheapest shipping, quite a bit more than "random Chinese seller".

Meanwhile certain stuff which is common and low spec like resistors and capacitors with larger tolerances, as well as things like switches, are fine if you get them from China (they're actually made there) as are low resolution displays, plus there are some pretty useful integrated circuits which are Chinese (say, if you want a simple and cheap USB-Serial adaptor to add basic USB comms to a circuit, just get a CH340g).

The fake parts problems tends to pop-up with more expensive parts like microcontrollers and microprocessors (plus the advertised specs of basically everything power storage and power generation are total bullshit).

So if all I need is a bunch of cheap parts or stuff actually made there (say, small displays) it's fine to use something like AliExpress and the cost of an order including shipment will probably be less than just the shipping costs for TME. However if I need stuff like microcontrollers or 1% resistors with non-standard resistance values, I'll try and come up with a bigger list of stuff I think I need and make a large enough order on TME that the shipping costs aren't a large part of the total cost.

Digikey is very expensive in comparison to other suppliers. I often use Mouser or RS and others via our purchase department. But sometimes, Amazon is just cheaper for small amounts. And sometimes, Amazon is actually cheaper than the big suppliers for unknown reasons. There are connectors I buy at Amazon in quantities of ten pairs, and they are cheaper than even larger bulk prices at the big suppliers.

I don't buy that stuff of Amazon, but Aliexpress is much cheaper than Mouser or Digikey for a lot of it especially if you just want an assortment of stuff for future projects instead of exact quantities for your BOM.