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).
It's simple, like what you do with online shopping, you don't blindly buy an item to an online store that hasn't got any reviews. It's nice that you have money-back protection and all that stuff when you use your credit cards in buying an item but the amount of fees you'll be paying is somewhat crazy + the late fees you might incur if you don't get to pay your bills on time. Personally, I think that the straightforward nature of bitcoin alleviates the need for intermediaries; you only need legitimate merchants on the other end of the spectrum and bitcoin can act as a currency flawlessly. Unfortunately though, legit merchants are rare since they are scared of bitcoin's volatility and whatnot.