Many developers learnt first to code on isolated machines - DOS ones if they are old enough - or within simple networks. DOS was a single process, single thread environment, almost always on isolated machines. Maybe a modem to connect to a BBS, LAN were much rarer. That meant developers had little to care about "the environment" their application where "living in". Once the application was started, it was the only one running (but some tricks like TSR) and could use all the resources available. Slowly, the environment changed.