Even building off sites like Newegg, it's hard to match specs with Dell. They build thousands at a time, and buy in bulk. I haven't done it recently, but the last time I tried, I took a Dell, and priced out the parts to buy off Newegg, and the Dell was much cheaper than I could build the same system for. Though, if they don't build what you want, you have to build it yourself. For a while they did more options for customization, and you could get most anything you wanted, but they have baked off that, as the people buying systems prefer it simple.
Though Dell (and others, I just use them as a named example so it can be fact checked) often has upgrades for more than the price. I've bought a laptop from Lenovo with 8G RAM. I paid $20 more for 1 DIMM, so I could put in my own 8G in the other slot, as the price of RAM was about 1/2 Lenovo's price for the upgrade.