Scottish has a trill, not a rhotic for R. Since r changed to l, it is rather obvious that it was pronounced at the tip of the tongue, not the back. Otherwise I'd have doubts that I'm confusing Scottish with an Irish accent.
Consider that next to perception (as mentioned in another answer), production might also play a role, especially for children (I wuv you vewy much and the like). I agree that it is awkward (also compare arkward vs awkward, such coincidences). In particular, the sequence of the tongue through the mouth stresses antagonistic muscles repeatedly in succession for those letters. Naturally, the tongue get's cramped only after the succession, but grama would obviously clash with a word that's reserved? On the other hand, glamour might be a glorious egg-corn.
At any rate I just wanted to note the trill or tap, which was alluded to in prior comments.