Why is Android phones' RAM capacity catching up with or even overtaking PCs'?
It depends on screen size and pixel density - so the minimum for 32 bit is close to 1.5 GB for 32 bit 2GB for 64 bit at the highest end - Section 7.6.1 of android Compatibility Definition
This size is only for the OS , radio (modem) is extra, which is not much anyway.
As discussed in this question 2 GB RAM or 3 GB RAM, does it really matter?, RAM requirements from OS per se are not high but as pointed out by Andy Yan in his answer - apps occupying more RAM with usage and apps with increasing functionalities (WhatsApp for instance ) tend to grow in size


But coming to your question, why in Android and why not in PCs , IMO, factors which are unique to Android


Multiple apps being open especially social apps in addition to standards mail, messaging , maps etc can lead to some apps being killed as RAM is less (not so on PCs) - poor user experience
Future platform releases - 4GB is strongly recommended ( see last section of CDD linked above ) . Compare with Windows minimum requirements - RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit). For Linux it would be far less 
Selling point, whether it matters in reality or not (on a 5 inch screen does HD make a difference ? Not to me, at least )