Because often the principles deduced from it involve more than just one "proof text" but synthesizing a number of different texts to produce a harmonious result. See here for Joseph Farah's article entitled Should Christians always obey government? And then he proceeds to try to refute the claim of Bible believing Christians who teach Romans 13 demands this.
But he knocks down a straw man. Even the thinkers who have the most fundamentalist approach on Romans 13 (Drs. John MacArthur and Gregg Frazer) don't claim this. Rather they claim SUBMISSION to the civil legitimacy of government is absolute. (That means rebellion is categorically forbidden).
Of course, they understand the competing verses and chapters of scripture like Acts 5:29 that say when the two conflict obey God not man. And the principle they deduce from putting the verses and chapters together is if government is ordering you to do something that the believer in good conscience thinks "sin," then disobey government. BUT, accept the civil legitimacy of the process when government comes along and punishes you for it.
Don't rebel or try to overthrow it. Rather work within the confines of the positive law for a solution, if you can get one. If not, then you'll just have to accept your punishment like a good martyr.
I did note when I presented at Gordon College on Dr. Frazer's book (which holds, among other things, that the American Revolution -- as all revolutions do -- violated Romans 13 and the rest of the Bible), that if objective truth can be found outside the four corners of the Bible, that could change the understanding. (New principles need to be synthesized in with the competing verses and chapters of scripture.)
Among others, the "Patriotic Preachers" (many of them unitarians and natural law believing rationalists) discovered a right to revolt against tyrants in nature via the use of reason. These preachers believed in a theistic natural law. That is, the God who authored scripture also authored the principles of nature discovered by reason.
So after finding a right in nature through the use of reason to rebel against tyrants, they then went to the competing verses and chapters of scripture with that truth and added a new element into the equation. This resulted in an understanding of Romans 13 where rebellion against tyrants was permitted.
Samuel West for instance, addressing the claim that the ruler St. Paul instructed believers to submit to was the pagan psychopath Nero, asserted that the right to rebel against tyrants was so clear a teaching that Paul might actually have meant Romans 13 to be satire.