Why yes, Slack does seem to be written poorly - just as poorly as every other web app that I use including VS Code, Discord, MS Teams, etc..
Maybe the Slack developers are just stupid, uneducated, malicious, poorly managed, or ambivalent (or all of the above) but the platform does to be conducive to creating clunky and bloated software.
While i agree with your overall point, i think that VS Code is one of the better (only?) examples of really good web technology based software. It's snappy, reliable and has very few bugs.
