It depends on the user. You could argue that many users in technical roles would see a professional interface, as one with less white space that allows for more

It depends on the user. You could argue that many users in technical roles would see a professional interface, as one with less white space that allows for more information on the screen at once. However, many users in non-technical roles actually prefer more white space. This is why the default options for newer software redesigns (e.g. Outlook) favor more white space, with a "compact" options in the settings for users who prefer more information. I think the Teams design team tried to compromise by making the interface approachable enough for the greatest number of users. But for one reason or another (strategy to differentiate from Slack, company DNA, or other reasons), they couldn't make the interface subjectively feel more casual and less professional. Everything from the name ("Microsoft Teams") to the logo (a bunch of people, versus the Slack logo with eyes and a mouth) is more serious, even if the team tried to soften it a bit by using less formal language in the interface.