Think about it this way, Just because the store across the street increases the price of a bottle of water to $20, doesn't make the $15 that the store down the road is charging, a good deal.
Relatively speaking, a prebuilt can be be cheaper than a DIY system, but that is only in comparison to the insane price gouging that is happening.
Both are bad choices at the moment.
Compared to a year ago, you are still paying much more whether you go the DIY route or buy a prebuilt system.
If you choose to go prebuilt now, you are shooting yourself in the foot twice as not only are you still paying a steep premium, but you are also giving up the far longer individual warranties, for what is often just a 1 year warranty that covers everything, but also often requires you to mail in the entire system at your own expense
When you build a PC,you get a 5-10 year warranty on the power supply, 3-5 year warranty on the motherboard, 5+ year warranty on the SSD, lifetime warranty on the RAM, 3 year warranty on the CPU, 3-5 year warranty on the hard drive, and 3+ year warranty on the GPU. If you buy a prebuilt system, you will be paying a price premium to essentially not have a warranty.
Most large system builders get their components for far less than a store like newegg or amazon would get them, as the system builder is getting components that do not need to have the cost of a warranty or any other after market services built into it. They should be able to offer a prebuilt system for less money than it would cost a customer to build their own system, an still make a decent profit, since the user building their own system has to pay multiple retail markups in addition to a price that covers a long warranty.
Over the past year prebuilt systems have also become much more expensive, just not as bad as when buying individual components.
The price markups are happening on the retail side and not the OEM side, thus the only thing really pushing those prices higher is an effort to maximize profits and charge the maximum that the market will handle.