It's all personal preference and a couple of keywords I'm going to use, you'll notice them.
1) Sense of owning: for some people including me, it's just feels nice and right to own a game physically. Being part of your little (or huge) collection. Having boxes that look nice together and all that, like when you collected cartridges back in the SNES era. The fact Switch games are actual cards instead of easily scratchable discs that will die on its own over time like all disc media is a huge plus, it's "more physical" if you get what I mean.
2)Resell value: If you hate the game you can always sell it, get money back and buy something else. You can even get more than half spent if you played for just a couple of hours or something. You can also just trade and that.
3)Some may agree some totally disagree but I feel more commited to finish and 100% my games in their physical cards. I don't feel digital games have that same kind of value.
So yeah, my main reason is solely collecting and appealing to the older age of games when we collected cartridges. I was born too late to collect SNES games (in my country SNES cartridges are expensive as HELL because resellers are the worst "It's a collectible and retro, man") but born just in time for Switch collecting.
Between spending money in digital games literally "buying air" I'd rather have a nice box with a literal physycal game card inside.