What % of billboards are owned vs leased by OOH ad agencies?

I live in California and am trying to get into the billboard investing business. Just doing some preliminary research as of now. I’m wondering what percentage of billboards are actually owned by large ad companies (Lamar/Clearchannel/Outfront etc) and what percentage are leased by those agencies from the landowner?

If most are owned by the agencies, why? Isn’t it more effective for the landowner to lease the billboard and take a cut of the revenue, and let the agency manage selling inventory?

Any info is appreciated. Thanks!