> Could you explain why Signal users are more careful than email users?
Sorry, I forgot to mention that phones are typically seen as more secure, and phones are the go-to operating systems that people use now, and are (usually) permanently switched on, so have to be secure since they are constantly exposed to the public Internet. (Yes, Windows can be seen as secure too, but IMHO phones are more secure. Windows is getting better over the years and have mitigated and patched a lot of the common vulns you do see).
> Aren't all Signal users also email users?
No. Email is often reached from many different OSes and environments. It is common and expected to see people logging into their Gmail from potentially compromised systems at work, or at Internet cafes. They just assume that whenever they login, the are 'secure' when in some cases the Internet cafe is logging everything or their employer has setup 'monitoring' software to ensure they are actually working and not dossing.
Signal: not so much. They have a single secure device that they use to communicate with, and since Signal is tied to a SIM: migrating your old Signal 'account' to a new SIM is impossible.