As I have understood, in order for one to be able to buy Altcoins, one must first own BTC or ETH. What is the reason for this?
Many of the larger and more established alt coins can get bought directly via USD / EUR as well. Kraken for example has fiat trading pairs for LTC, XMR, XRP and a few other alts. Bitstamp offers fiat trading pairs for XRP and LTC. I guess a lot of alts don't have enough volume to warrant dedicated fiat trading pairs however, so naturally BTC or ETH are the better fit.
Also note that the exchanges with the largest selection of alt coins usually don't have a direct connection to the traditional banking system.