Because in most markets where people expect great appreciation (or depreciation) in real estate values, there is inherent volatility in that market for the precisely the same reasons as anticipated price movements. 

Over time and as you scale to more properties, you get closer to the averages but single properties will have significant variability.It's so much easier for me to buy value adds and force appreciation and have large cushions against precision in underwriting a deal.  

See then if you can formulate a more precise plan for building longterm wealth.

Now, I understand that many people don't use accurate numbers when asking a question on a public forum, but I feel that doing so will help the BP community answer my question more precisely. 

That's precisely what I am aiming to do.

This is precisely why Cap rate as an indicator of the worth of an investment opportunity can be very misleading indeed!

I just heard from a guy who owns over 650 empty lots but he has a very precise system for buying at auction and strategy for selling them.  

Best to know how to calculate what the money is costing you so you can make a more precise and educated decision.John Corey

However to be honest, these are just rough estimates and you really don't know with any precision the capital expenditures that will accrue once you first take possession of the property. 

Also, I have little to none interest in NY (where I live) or NJ (where the condo is), precisely for the reasons you mentioned (high taxes!).