- E-mail always has a round-trip latency attached, even the best systems will have at least a minute, and you have to do insane amounts of work to keep up with "deliverability" as automated anti-spam systems can and will detect high frequency of incoming emails and downgrade your score, not to mention you have to deal with bullshit like contacting providers that downgraded or outright black-holed you.

- Uploading or sending attachments is a hit-and-miss, there still are providers limiting attachment size to 1MB (looking at you web.de) or with overzealous "security software".

- People will readily ignore the "do not reply below this line" and reply inline, which software like JIRA SD won't recognize or garble up.

- On mobile, people will have to switch between apps, which more often than not even on flagship devices leads to Android killing off the apps between switching for "memory efficiency" and 10+ seconds of load time after each switch.

On-site chat has none of these problems.