But what about the Atlantic (both Africa & the Americas), why is there no imposing mountain ranges, volcanoes, & earthquakes on both sides?

Well, that's because the sea floor of the Atlantic is not active.

Meaning, there's no "sea floor spreading" & hence, there are no deep trenches off the coast of both continents unlike in the Pacific where sea floor spreading is so active that there exists deep oceanic trenches off the coast of the landmasses of Asia Pacific & the Americas which I already mentioned awhile back.

Hence, it's to no surprise that you can walk several meters [or even kilometers!] off the beaches of the Atlantic since there are no deep trenches below whereas in the Pacific, the beaches slopes deeply rightaway 'coz of the deep trenches below.