AlphaZero is not active, but you can play Leela LC0
Why is there no option to play AlphaZero online or by a able chess program?

Perhaps because it's owners are snobs, since the engine is best in human and computer history.
And perhaps he thinks us little people are unworthy to address the (Chess) King?
The programs were basically a tech demo, have been updated at least once since then. It also does not run on commodity hardware, requiring specialized hardware to run on, so most people couldn't run it if they wanted to.
Perhaps because it's owners are snobs, since the engine is best in human and computer history.
And perhaps he thinks us little people are unworthy to address the (Chess) King?