it's cheaper to build it and also more fun....
and i dont think you can change parts inside a computer if you bought it from a company...unless you change parts from the same company. or you will lose warranty
It's way better to build a computer. Much more fun and cost effective. Also you can customize all the parts to your liking and it will be easier to replace or upgrade.
it is very fun to build your own pc, it isn't that complicated, you can customize it for what you want, also, you can do it for the cost of the parts, nothing else!