Oh come on. How did you post this, with magic? If you have a PC then you have those parts, just buy what you need and take whatever you can from your old system. Key board, mouse, really?
It doesn't take much money to build a PC if you already have your basic components (case, PSU, KB/M, etc). Most people think it's super expensive because they look at pre made PCs like alienware which charge insane amounts of money and take advantage of people's lack of PC knowledge.
If you want a gaming PC, read up on it, look at some guides, and buy the parts you need.
It isn't difficult.
I'm plenty familiar with porting parts from other pcs. But for many people (most?) that isn't a feasible idea. First comes the knowledge to do so (maybe not so much an issue with input devices or monitor). And the expectation that someone should spend the time/effort to acquire such knowledge simply to play a game is ridiculous.
Second, even if you have the knowledge, is the availability of parts to scavenge. Unless you are single, or the only one in the household who uses the current computer**, then the only place to scavenge is acquiring second-hand parts (again, time and expense, albeit the expense would be less).
** Most people I know who share a pc with others wouldn't benefit from transforming it into a gaming pc anyway since its availability for lengthy gaming sessions would be quite limited (or lead to ill will with those they're supposed to be sharing it with).