The Apple ][ had expansion slots.
"Apple co-founder Steve Wozniak was a strong believer in hardware expandability, and he endowed the Apple II with luxurious expandability [....] Steve [Jobs] was reportedly against having slots in the Apple II back in the days of yore, and felt even stronger about slots for the Mac. He decreed that the Macintosh would remain perpetually bereft of slots, enclosed in a tightly sealed case, with only the limited expandability of the two serial ports."
"Burrell was afraid the 128Kbyte Mac would seem inadequate [...] He realized that he could support 256Kbit RAM chips simply by routing a few extra lines on the PC board,
But once again, Steve Jobs objected, because he didn't like the idea of customers mucking with the innards of their computer. He would also rather have them buy a new 512K Mac instead of them buying more RAM from a third-party."