The funny thing is that the early malls were like that. The word "mall" originally just meant an open space, like the National Mall in DC or the "Cuba Mall" that I grew up near in Wellington, New Zealand.

The modern shopping mall is distinguished by a single financial entity which owns the land/buildings, provides shared infrastructure (e.g. parking/power), and rents it all out as a package to many mostly small tenants. Enclosure is an optional feature. Later in life I lived near one of the earliest US malls built on this model - Shoppers World in Framingham, MA. It was not enclosed. It was basically a ring (or maybe more of a figure eight) with two stories of balconies over a shared courtyard. The familiar enclosed structure came later.

Ironically, Shoppers World itself became unable to compete with the newer enclosed Natick Mall across the street, so it was torn down. Now the name persists, but it's basically a big parking lot with a dozen or so isolated big-box stores around the periphery.