They needed enough cash flow to keep the dr office open as well so things could have been tight even with the money in the safe.
I still don't understand why Mark told police they were struggling and living paycheck-to-paycheck, but then admitted to having $40,000 cash in a safe? And did he not think they'd find out about the tax lien? Why would he admit to having that 40 K in cash? What did that accomplish?