Why would anyone use iMessage/SMS on a corporate/work phone, if the corporate messenger is already installed?
Because that OS is far too old/EOL? Nobody bothers writing software for ancient platforms like Android 5, and why would they? Just so like 0.000001% of their userbase could use it. It doesn't make any sense for them to do it.
Because most of those people are idiots.
Maybe they're just watching it for the plot.
And then people wonder why they are so lonely... Geniuses