Why do credit card forms ask for Visa, MasterCard, etc.?

You can tell if it's a Visa or MasterCard based on the number it starts with, i.e. 4 for Visa, 5 for MasterCard. Why do most billing forms request the type of card?

• It's not a barrier for bots (to my knowledge).
• It's redundant information.
• It's an additional button to click for someone about to pay for something.

Why? If it's for consistency of experience for users:

"Wait, why wasn't I asked what type of card I had? Everyone always asks that! Something's fishy here."

Why not just show them "Visa" (or whatever card symbol that they have) as they start typing?