Once you get out farther from civilization these signs become less common and there are more situations where they can get removed.

Most cheap hired labor doesn't realize you shouldn't dig up the market posts and throw them in the dumpster on construction sites. Add a little bit of rain and suddenly the contractor showing up with an excavator that was told everything was properly marked, and can see markings in other places but not where they are digging, and disaster occurs.

Also soils aren't static, I've been on sites where the midpoint of a buried telecommunications cable had drifted over 8 feet from where the markers were a few hundred yards apart. Just looking at the markers and assuming a straight line isn't a safe bet. Looking at the fence rows to the left and right of the cable and you could see a matching bow in the fence rows.

Just call 811 and let the utility figure it out.