I am not in a community state, so why would I need my spouse's agi if we are filing separately?

Even if you're Married Filing Separately, you'll still need to enter a bit of info about your spouse.

Entering your 2023 AGI for both of you  (which would be the same, if you filed jointly last year) for your MFS return is just used as a way to identify you to the IRS.  It's not used in any calculations.

If you're using TurboTax Desktop, you could enter both of your incomes/credits, then go to FORMS, Open Form, type in What-If and a worksheet will open showing amounts in the first column for MFJ.  Check the box at the top, and the worksheet will fill in comparing you and your spouses income, deductions, refunds, etc.

@rjkovach2