I think that building a PC is better value for money and you have the fun of building the PC for yourself and choosing all of the parts yourself for a truly customisable feeling. Another reason why I personally think that building a PC is better because you learn a lot from doing all of this and as a tech savvy 13 year old I really enjoy doing the research and learning on building a PC.
Building a PC may have its benefits, but buying one is much more reliable. The creators of the PC's you buy are technology exports, and they know the proper way to design a computer. It is possible that you may make a mistake, and because you're dealing with different companies for the various parts, it'll be complicated to figure the situation out (1). Also, buying a computer is quicker, and it requires much less effort. I do not want to figure out how to build a computer, as I want it now. The price of building versus buying is close enough that the effort is not worth it. Thanks for debating me!
There may be a larger change that a bought PC is more reliable, but a custom built PC is less likely to slow down over time due to annoying bloatware and free trial software that uses up more space and slows down your bought PC. On the other hand a custom built one will not come with any of this useless software, unless you install it yourself which is highly unlikely. By not having any of this unwanted software you can feel better as your anti-virus won't suddenly stop working because of a silly trial as you have actually bought a reliable anti-virus program that actually works.
Another reason why building is better is that you have fun doing it. Yes there is fun from unboxing a pre-built PC but you have even more fun building it yourself. I must admit it takes more time but you can enjoy the benefits that you get from the custom parts that you have chosen. And most of all, who doesn't like having fun??