For most people it doesn't serve a purpose outside of what they're reading (I own a few bitcoin that I mined back in 09, but I'm never really compelled to use them for anything due to headaches and fees trying to spend them).
Bitcoin has problems but spending UX for online transactions really isn't one of them.
> Credit cards you have to (optionally) make sure the site won't steal your number,
Nope. If it's stolen, you get issued a new card and all the old transactions are refunded.
> go get your physical wallet, fill out a rather long form
Browser integration makes it easy to fill out these forms. I don't use it, but I've seen people who do.
> and then occasionally call the issuer because they decided someone might have stolen your card and won't let the transaction go through because it's big.
In my experience card companies let the transaction go through even if it's big. Then they call you. Mine sends me a text saying "Was this you? Type 1 for yes" and that's the end of it. I like that. It's a good thing.
Personally, I'm relatively sure I will never use bitcoin, because I pay my groceries with cash and my rent per wire transfer, and other than that I buy stuff off the internet maybe twice a year, so entering my credit card details isn't quite that annoying. I realize my spending habits are somewhat unusual, though.
I go to a website and click a "Pay with Bitcoin" button and am taken to a page where I have to send X BTC to Y address. Ok, easy to copy and paste them, I've done this plenty of times in the past.
And then I see "don't pay directly from an exchange!" because they take longer to send transactions or don't pay enough in transaction fees or something. I used a regular local wallet for a time but have been using Coinbase as my wallet ever since, and now that won't work. I've normally been able to pay via Coinbase very easily. Let's just download the blockchain...
120 GB? Well, better grab an old external hard drive...
26 hours to sync, despite my 200 Mbps cable connection? I guess I'm not paying with Bitcoin today.
I can definitely see how paying with Bitcoin could be annoying to an average consumer nowadays.