Why bot doesn't trigger the first message( welcome message) for the first time user deployed in MS Teams

Hello Team
the client’s bot is deployed in MS Teams,
when a new user opens the bot in MS Teams for the first time the bot doesn’t trigger the first message ( welcome message ).

what client’s ask is just like in web where the bot triggers the first message( welcome message ) after opening the bot similarly in MS Teams should happen.

can you please help me to understand WHY this is so ?
is there any approach we can trigger a proactive Welcome message ( or notification ) for first time user.

Same as Whatsapp, the intent needs to be triggered by an utterance.

yeah that is known.
wants to know WHY it is so.
similar to web, why it doesn’t trigger automatically for first time user when deployed in channel (MS Teams)