How is this the most costly way?

Opportunity cost of the additional travel time[1] that you have to spend because you don't have a car.

[1] Yes, I'm aware of european cities where cars aren't necessary or are actually slower than public transit. That's not applicable to most of the US though.

I wonder how many instacart/uber orders I would need to have to offset the cost of a car, assuming I can bike to most of my needs.

The only reason I have a car is because there are some specialized transportation needs (towing) that I cannot get from my bike. I use my bike for everything from hardware to Costco to groceries to child care to ... lots of stuff.

You're in the minority. Most people cannot use a bike for this stuff. They live too far away or the roads and/or terrain are not suitable for bikes.

> I wonder how many instacart/uber orders I would need to have to offset the cost of a car, assuming I can bike to most of my needs.

Not that much. One site[1] lists the TCO of a compact car at around $33k/year if you drive it for 15k mi/year for 5 years. That works out to $550/month. Of course, if you're comparing this to getting ubers, there's no way that you'll be driving anywhere near 15k mi/year, so the TCO of a comparable car is probably $450/month. That's a lot of money to spend on uber/instacart, but keep in mind that if you have a modest commute of $20 each way, that only works out to 11 days a week, or half the working days.

[1] https://www.kbb.com/new-cars/total-cost-of-ownership/

I've got an ebike that takes me anywhere within ~10 miles easily, including picking up/drop off kids), and I'm a remote worker (though I could bike to the office easily.)

I have a friend whose only reasonable option to commute to/from work is uber/lyft. He spends more each month on that than I do on my car loan. He can't afford to make a downpayment for a car loan of his own, because he is spending that money on uber/lyft. This is a vicious and familiar cycle in America.