A chat program? Easy. A chat program with 270 million monthly active users? Slightly harder, but sure, still easy. A chat program with 270 million monthly active users that has to integrate the entire Microsoft ecosystem [sharepoint, outlook mail + calendar + contacts, yammer, O365 groups, etc] because that is what their enterprise customers demand? Slightly less easy. A chat program with 270 million monthly active users that really isn't just a chat program but rather an entire chat + teleconferencing + screen sharing + MS O365 app platform which has to deal with B2B AAD authentication cross organization, legacy on-premise Exchange servers setup in a hybrid environment, excessive levels of enterprise-controlled configuration and policies, compliance (Purview, IB, retention policies, etc), legal requirements needed to allow Teams to be used as part of the Microsoft 365 Government offering, and many many more niche requirements/features? I suspect it's hard.
I don't particularly like the software (and had many issues with it myself at my prior job) but MS Teams is not realistically _just_ a "chat program" that is easy to build. If you are a company just looking for a good chat experience, I have no idea why you would not use Slack. However, if you are one of the countless companies who have already bought into the entire Microsoft O365+outlook+AAD+sharepoint ecosystem, Teams is probably a good choice, as it is very seamlessly integrated.