RPI 35 + power supply 10 + power switch 10 + sd card 10 + case 10 + PS3 wireless controller 20 = 95$, but you have a better device than the classic console.
"Better" is subjective. (Note: I don't own a SNES Classic, own original hardware, and do emulate elsewhere, but bear with me). The SNES classic is literal plug and play for anyone, regardless of technical aptitude (the RPi is not, even if you handed one pre-configured to someone, and I speak from experience as a 20+ year Linux sysadmin veteran who has built these for friends, and seen them still bamboozled).
Additionally, Nintendo's emulation quality is excellent - only Byuu's Higan/bsnes in accuracy mode is close to being on par, and requires far more processing power than the humble RPi can output.
I don't get it because I can actually buy an original nes or snes console and a multicart for less money
I can't. SNES consoles average around $100 where I am. Good quality multicarts well over $100 (ignoring the SuperFX2 incompatibility, which adds cost again).
Again, I do prefer (and own and use) real hardware, do prefer Linux and love RPi hardware, and am not buying a SNES Mini. But I 100% understand why others would. The potential market for them is large, and their value is fantastic for many people who simply don't want to deal with the complexity (and it is complex for non-technical folk) of hooking up old consoles, sourcing old TVs, or dealing with Raspberry Pis.
Preferences are preferences. But there's clearly a huge market for these devices, and it's easy to see why.