The other question I have is that if we can combine building and land in a bulk sale, then why does the IRS tell us to separate them? The IRS pub https://www.irs.gov/pub/irs-pdf/i4797.pdf says:

Depreciable Property and Other Property Disposed of in the Same Transaction

If you disposed of both depreciable property and other property (for example, a building and land) in the same transaction and realized a gain, you must allocate the amount realized between the two types of property based on their respective fair market values (FMVs) to figure the part of the gain to be recaptured as ordinary income because of depreciation. The disposition of each type of property is reported separately in the appropriate part of Form 4797. For example, for property held more than 1 year, report the sale of a building in Part III and the land in Part I.