Is there a reason why such a high % of indie games drop cards?

Because they get money from every card sale.

^ +1 to both comments.

• money from card trades and sells.
  Plus I would guess that as they are usually made by a smaller, tight group, it's easier to make (or import) art to the cards. Or maybe just money, but eh :)

to make me happy!

Cards start dropping after 2 hours, so there is a choice refund or cards. So games with cards have less refunds.

Selling cards gives at least 0,01 Euro (or 10%) per sale to the game creator. If you have sold 1 000 copies for full price, gave away 200 and sold another 10 000 on 1$ tier Humble Bundle you'll get about 7 000 Euros from Steam sales, 2 000 from HB and about 400 Euro from cards. (assuming 10 cards in game and 10 Euro price) This means that you just improved your earnings by about 4,4%.

Also sometimes people buy games for the cards. It allows you to increase sales by 0,5-1%.

So in total as an Indie developer you'll get about 5% more income if you have cards (not counting refunds - cards won't save crappy game, but might be a life or death for average in quality but short game)

Because it gives people an excuse to buy them, which means more money for them.

Are trading cards reserved for AAA games? Are they a bad thing?

For some developers is the main source of income, for example Sometimes You or Digital Homicide Studios give away thousands of copies of their trash games expecting to earn something from trading cards sales.