i receive social security direct deposit so why is my stimulus being sent by mail?

I got mine today, the old fashioned way. .by US Mail

And what makes you think it will be? Because it will be direct deposited UNLESS you filed federal income tax returns and didn't use direct deposit for a tax refund.  The reason?  They are ONLY using social security data for those who didn't file federal income tax returns.

social security is direct deposit!!!!!!!!!!!!

Depends on how you got the tax refund.

It's being handled by the IRS.  Unless you've gotten a tax refund, direct deposited for 2018 or 2019 - the check will be mailed.  You could have gone on line and requested direct deposit , but I guess you missed the window for that.  

Do you get a tax return?

If you do and it is direct deposited then the stimulus will be direct deposited into your account.

If you get a tax return by mail or do not get one at all then the stimulus check will be sent by mail.