As marshray said below, Tor doesn't generally try to hide the fact that you're using Tor, only what you're doing with it. This is complicated by the very active research on obfuscating methods for accessing Tor, but those methods are mostly trying to prevent automated large-scale detection in real time, in order to evade blocking by national firewalls. They aren't necessarily trying to prevent more manual or after-the-fact forensics that might confirm that a particular person was using Tor.

To be clear, the threat models of the obfuscating transports can vary, so what I've described is just a trend in emphasis, not necessarily a suggestion that nobody ever cares about obfuscation-in-retrospect. But the history of that work is around censorship circumvention, which is often a slightly different goal (with slightly different priorities) than confidentiality.

For example, I've heard people who work on obfuscation talk about how it would be good if something required an expensive calculation in order to distinguish from other traffic types. They care about this because a national firewall may not have sufficient capacity to do this in real time.

Depending a lot on your threat model, Tor might still be a benefit even if people do know you are using it, supposing that they don't know for what.