Flat maps are rather easier to photocopy.
I know, very pithy, but unfortunately the technology isn't always there yet in districts to make online-based instruction fully feasible - the schools at this point are probably almost all set up to allow teachers to use an online (or offline, but still computerized) resource for their front-of-classroom instruction, and some have enough computers in classrooms to allow students to group up and do some work there on their own, but that's far from a sure thing. And nationally, as of 2013
one in four American households did not have an internet connection, and the poorer the household and the lower the educational achievement of the householder (i.e. the parent), the less likely it is that they have a computer and internet at home. In Mississippi, only about 62% of households had high speed internet [note: in the first few tables in the pdf, the percentages across various demographics with high speed internet were consistently only a couple points lower than the percentages with any internet access at all]. When kids are learning about the names of continents or the location of countries in third grade social studies, it's hardly fair to expect a large chunk of them to have to go to the library to find a computer to do their homework instead of the teachers handing out worksheets. At some point, paper maps do have to enter the equation, and unfortunately I think it ends up being most necessary with the younger students.
As I said above, though, I would prefer if schools used one of the various interrupted projections, which cut down on landmass distortion and make the "unpeeling" aspect clearer. Mercator, etc. can come later, when students are advanced enough to understand the basics of some of the math behind them.