however most already sign contract before hand.
"But I've got a contract!" That's funny.
1. Many businesses go out of business, and in that event rents aren't paid and leases are unenforceable. Those particular contracts can be worth less than the paper they're printed on.
2. Every month a certain percentage of leases comes up for renewal.
3. Some leases have operable exit clauses in various situations.
4. Tenants frequently renegotiate leases. That's going on right now, and (in fact) the government is insisting on it -- that landlords pass along property tax concessions, for example. The government might not only insist but legally require it. Ordinarily landlords would try to keep property tax savings. Not now. Landlords that refuse to renegotiate leases are could see #1 or #2 occur (#2 with non-renewal).
5. Landlords are unable or less able to roll over their debt loads when their properties are worth less, and they're now worth less. Lenders and bond markets constantly reevaluate risks.