1) Sounds produced by analog equipment cannot be reproduced exactly in software. You can get very close but so far nobody has gotten it 100% right.

2) The physical controls on hardware make the process of creating music more tactile / intuitive / emotional. It's faster to reach and turn a knob than to click on the knob and drag it -- maybe only by a little but when you do that thousands of times it adds up.

3) The constraints of using hardware can be a useful artistic limitation, that actually increases creative output.