I worked in a company that also sold a collab solution and we had technical champions all over the place that 1000% agreed that our product was better than Teams in every single metric, including performance, UX and productivity. Yet they couldn't secure a budget since the higher-ups knew they got Teams for free in their E5 license.
Not sure you realise but in the corporate world it's about politics and money. People (above entry-level staff) do things to look good to their boss. That's it! And saving money is a great way of getting promoted: "Look boss, I just saved us $500k a year in license fees!".
Saving $X per year using a "free" tool from Microsoft will always trump anything you pay for especially if you are all in on Azure, O365 already. It's a no-brainer.
Not only that, once the decision is made, it will likely never be changed until the higher-up that made the decision moves on, quits, or is fired, no matter how wrong or bad the decision was (well, within reason, of course!)
I'd love it to be as simple as making a good case for the competition, and I've had to make that case many times over the years, but the reality is that a bundled product from Microsoft will win in a place that uses other Microsoft stuff, vs a paid product thats 100x better, faster, stronger, whatever.
It's because it's being sold to managers and executives who don't actually end up using it, and never have to deal with the consequences of buying it.
individuals are not pricing that in. coordination is needed. that's why we regulate the market