Almost all verbs ending in -er are conjugated the same, and parler is one of them. For many of them like écouter, danser, présenter, it would change the pronunciation significantly to drop the -e for “je” conjugation. It makes more sense to keep the -e on the “je” (and il/elle/on) conjugation for all regular -er verbs than to have so many “exceptions to the rule” for the pronunciation of what would be “j’écout” or “je dans”

The [o] did not “disappear”, it eroded into a schwa sound which was still pronounced in Middle French. That vowel is what the e in those verb conjugation actually represents. You see, with the exception of various fixes that do not even necessarily reflect pronunciation better (like the plural of nouns in -nt, which until the early 18th century were spelled -ns in the plural), French spelling corresponds well to what was spoken around (if memory serves) 1600.

In any case, by the mid- to late 17th century, the schwa was dropped very frequently in the spoken language and now is dropped almost everywhere.