(1) I don't have Jira, (2) I don't want to fill out a SLA ticket, (3) I don't use Teams, (4) I don't know your phone number and/or prefer to deal with things in writing.

Email works because: (a) it is ubiquitous, (b) you don't have to pay for some proprietary software to use it, (c) you remain in control of your data (no IM messages suddenly disappearing), (d) you have a permanent, local, copy of what was said in writing, (e) it's often the standard court-recognised form of communication, other than post, for things that matter legally (e.g. sending notices).

That's not to say that email isn't without many defects. But it's still the best we have for many work-related use cases.