Why build a product on MAUI when Microsoft aren't too sure about it.
Microsoft Teams was released in March 2017. .NET MAUI was released in May 2022. In 2021, Microsoft replaced Angular with React and moved away from Electron to WebView2 (using the OS' built-in renderer rather than a bundled Chromium). So even the rewrite was a year before MAUI (and they probably started the rewrite before 2021). Plus, part of the point of using React there was that they could basically replace Angular bit by bit.
Microsoft Teams is just older than MAUI. It's like asking why Hadoop is written in Java and not Go or Rust or why Kafka is written in Scala and not Kotlin. Kafka was open sourced in January 2011 and Kotlin came out in July 2011. Kotlin wasn't an option given that they were developing it years before the language was released.
That's not to say that Microsoft's attitude toward MAUI doesn't leave concerns. There was some news a while back about a bunch of layoffs around MAUI. It's always concerning when there doesn't appear to be any dog-fooding going on - is this just some junk they're throwing at us that they don't want to use? I think some hesitation also comes from the Blazor side where it's looking like Microsoft doesn't really see Blazor as a React competitor so much as a way for internal company apps to be made quickly - in contrast to the Google IO presentations on WASM support for Dart/Flutter where they were emphasizing better-than-JS performance.
That said, Microsoft hasn't really released a lot of new (green field) stuff over the past 2-3 years. What product should they have made in MAUI, but didn't? You can't say Teams because that was a giant product way before MAUI even existed. Most of what Microsoft is doing is work on existing products - things they released before 2023/2024 and were in development before MAUI existed. Flutter had a 5 year head start on MAUI.
But there certainly is a feeling that Microsoft doesn't feel committed to it or at least not enough to put its weight behind it.