Speed Limits Only Apply to Some?

I was recently driving on Interstate-99, heading up to Happy Valley and had the old Mustang, windows down, cassette player cranking some old New York Dolls, proud of how the “Miranda” (yup, I name my car!) was cruising along at the new posted 70-mile per hour speed limit.  Then lo and behold, I glance to see a big old Buick SUV literally on my bumper!!  Mr. “I’m Important and Running Late” zipped past me like I was made of cement!  Oh, but this wasn’t the end of it!!! Numerous cars and trucks from places all over..Kentucky, Connecticut, New York, Virginia..(see, I was reading A LOT of license plates as the sped past me)..were I guess, oblivious to the LEGAL, increased, speed limit!!  When it was 55, folks went 65, when 65 go 70, now 70…how about 80??!!  I even saw one with a child in a car seat!!!!  I guess my question is..WHY bother posting one at all??!!  Because obviously it only applies to SOME PEOPLE!  I even sped up to 75 myself for a short stretch to see if maybe my speedometer was off..wouldn’t know, because that lady from Ohio with the “I’m for Hillary” sticker just flew past me like my car was in Park!!