That's incorrect.  Depreciation isn't optional, that's a common misconception.  Or at least if you don't claim it, the issue is that you are still subject to recapture, and so for that reason it is generally said to be required.  It wasn't devised by the IRS, it is defined by tax law under § 1245 and § 1250.  The reason we have to recapture depreciation even if it wasn't actually claimed is § 1250(b)(3) and § 1245(a)(2) and the terminology "allowed or allowable".  Notice that "allowed or allowable" is repeated multiple times in the tax code, and in IRS publication 544, for example:

"Depreciation allowed or allowable. The greater of the depreciation allowed or allowable is generally the amount to use in figuring the part of gain to report as ordinary income. ... If you did not take any deduction at all for depreciation, your adjustments to basis for depreciation allowable are figured by using the straight-line method."

This is a significant and common issue and they were misinformed or unaware of the depreciation requirement.  And then when they go to sell the property they face this issue of depreciation recapture of the "allowed or allowable" depreciation even though they failed to claim it for all the past years should have.  To address this situation, the IRS established that this mistake can be fixed with a DCN 107 accounting method change using form 3115.