For digital synths, yeah they can be fully emulated afaik. With the actual hardware unit, you get the knobs and keys, sure, but it's basically an embedded device, the OS and synth functions are tightly integrated. Which means nothing competing for the scheduler, latency practically 0. Plus they come with all the standard inputs and outputs needed for performance over a PA. By comparison a laptop is a bit of a liability. Who knows when some system service is gonna activate and add latency, or cause a freeze, or stall the audio driver, or something. Plus you then have to lug around all these adaptors and cables and external devices, for the I/O that you need that isn't built into a laptop. If you're a performance artist anyway, it's not as big a deal in the studio.
For analog synths, well they can also be emulated quite accurately, but the way they produce sound is just fundamentally different to digital synthesis. They're directly manipulating the voltage of current to get oscillation, and then manipulate that signal to drive the speaker and produce sound. This process can be emulated but it doesn't produce the same output in a physical sense. It's questionable whether a typical audience would notice the difference I guess, but there's something quite raw and visceral about using an analog synth.
For analog synths, well they can also be emulated quite accurately, but the way they produce sound is just fundamentally different to digital synthesis. They're directly manipulating the voltage of current to get oscillation, and then manipulate that signal to drive the speaker and produce sound. This process can be emulated but it doesn't produce the same output in a physical sense. It's questionable whether a typical audience would notice the difference I guess, but there's something quite raw and visceral about using an analog synth.