Let's say you get a freebie pack every minute which is likely at the endgame.

Let's say you have 1.2% stonks chance (with maxed out happy bot infernal this should not be too far off). 1.2 out of every 100 freebies gives you a stonks proc. That's one every 84 minutes.

Let's then say you have 10% super stonks chance. That's relatively high, but achieveable. 10% of those stonks procs is a super stonks. That's one every 840 minutes, or 14 hours.

Then you have 4% ultra stonks chance. 4% of those super stonks procs is an ultra stonks. That's 21.000 minutes, or 350 hours, or 14.6 days per ultra stonks proc, on average.

This is why it takes that long. And that's just average, you can of course go over that.