I believe the thinking is there are broadly two groups of users, with each group having a distinct approach or distinct needs to money management/budgeting and they are trying to satisfy both.
One approach (your approach) is for your total budget to be implicitly defined by the amount of money in your account at the start of the month. To make summary work for you, you need to avoid explicitly specifying an overall budget. But I think (although not entirely sure) that you can still use category budgets and left to spend will still be calculated based on your balance, as you wish. You just need to avoid explicitly setting overall budget.
The other approach (explicitly set an overall budget) is more suitable for people who use Monzo as their main bank account and so may have a lot more money in their account than they want to spend in a single month. For them, it’s better for left to spend to be calculated from an specified overall budget rather than from their account balance.
EDIT - Second approach also useful for people whose income is irregular or comes in multiple times a month (and so total budget can’t be implicitly defined by account balance at start of month).
I think what Monzo are trying to do is commendable (satisfy two camps) but they are not communicating well enough how the summary tab works and how its behaviour changes depending on whether or not you explicitly set an overall budget. The distinction really needs to be signposted effectively within the app. I’ve had to dig around the community and play around with the options in the app to understand what’s going on. That shouldn’t be necessary.
To see the excluded spending section just temporarily mark one your transactions as excluded and then switch back to the summary tab.