The BEST reason for building a PC:
An estimated 50 million tons of E-waste (electronic waste) are produced each year. The USA discards 30 million computers each year and 100 million phones are disposed of in Europe each year. The Environmental Protection Agency estimates that only 15-20% of e-waste is recycled, the rest of these electronics go directly into landfills and incinerators (Wikipedia). Professor Huo Xia tested 165 children for concentrations of lead in their blood. 82% of the Guiyu children had blood/lead levels of more than 100. Anything above that figure is considered unsafe by international health experts. The average reading for the group was 149 (Wikipedia).
Average life expectancy of computer devices (mostly due to heat)
PC can last as long as you want it to because you can do a much better job cooling AND because ALL parts can be replaced.
Notebook 3 to 5 years - Quite a few parts can be replaced but many users don't realize when the little fan(s) stops spinning how important it is.
Tablet 2 to 3 years
Smartphone less than 2 years (21 months average)
I suppose there will always be those who don't care about any of this.