[ry0ohki]

I'd say in part because you want to give people a UX they are comfortable with. Sites for years have been asking for "Card type", and so a user may suspect something is wrong if they don't need to enter it and show no indication you know what it is.

[eli]

That seems unlikely, but I've certainly been wrong about UI questions before. I'd be very curious if anyone has any data supporting this idea.

[qq66]

No data, but as a user I do expect to see a card type selector or indicator, even though I rationally know its superfluous.

[zorpner]

Unfortunately I don't have actual numbers for you, but a company I used to work for that did a fair bit of CPO had good data that removing that selector reduced conversions (the most interesting thing to come out of that study was that, at the time and with our client base, removing the actual card logos (e.g. leaving only the textual names) increased conversions).

[xur17]

I've seen sites (can't remember which ones off the top of my head) that do autodetect your credit card type while typing it in. I wasn't phased by it.