Why not? When you physically had to carry all of them around, it was simply impractical to have 20 different cards.
If you can add all of them to a tiny device, and they all come from just a few banks that provide a unified interface to manage 10 cards at a time, and/or set them all up for auto-pay, then why wouldn't you want to make the most out of all the offers and bonuses?
The entire system of credit card tie-ins is based on predatory debt practices. It's ingrained in US culture, but a lot of people on HN probably see it as a problem that shouldn't even have to be addressed in the first place. Facilitating the growth of the credit card ecosystem sounds like asking for trouble.
There are hardware limits on how many you can put in an iPhone, just like there are limits to how many you can stuff into a wallet. Most wallets won't fit 20 cards, either.
I fail to understand how can anyone claim that an 8-card limit in a digital wallet on a 32GB+ device is reasonable, when you can literally store more cards than that even in an average physical wallet.
And, again, Android's NFC and Google Pay have no such limits.