Because it comes bundled with Office 365 which they pay for, so it's "free". According to Slack, that's a company unfairly exploiting their monopoly ( on business productivity tools) and i agree.
There is an official deb package for Ubuntu. I installed it without paying for Office 365. Never actually used it so far though. Will it ask me to pay when I try to?
p.s. IMHO Slack integrations is the worst thing ever - usually abused to death and never remotely useful.
It’d be very cool if they could!