Fair enough on the 1st point.I can attest that game engines can and are being used as games by many, many people.
As for the trading cards limitation, it's a thing Steam does for its freebies. Every game drops like 3 or 4 randomly as you play them. Freebies drop none. I suppose it's because these this can be traded for [Steam wallet] money.
I am aware that freebies claimed directly from Steam don't drop cards, but I've never seen more or less the same version next to a free one. Also, the cards aren't worth the price, unless whoever buys that gets very lucky with one specific 15$ foil card. Weird, but whatever.