The answer is that it often is not, at least for most people, most of the time. Eventually it will be the best payment system, but it needs to develop further, gain more adoption, and get over the scaling obstacles. Even so, Bitcoin is already the best payment system for certain use-cases, such as sending money across borders, or receiving money from someone you don't trust (which helps further in cross-border transactions).
Being trustless is nice, but how would I go about buying an online item if I can't trust that it won't arrive broken and that I can never get my money back? So then there will always have to be employees from whatever website to be able to handle claims and escrow reliably...which means if they go against you, you don't really have an option to go to a bank to get your money back (not that a bank is even helpful with that either).