When my eldest was a baby, under 12 months, but probably not by much, I was visiting with a friend who was a mother of 5, and I know (now) is very gifted herself. My daughter babbled and she said "Oh she's saying "blah blah"" and started talking back to her. I remember politely nodding or smiling, and thinking she was completely crazy to think my baby was talking.
Some years later, in a park with my first two children, I was chatting with a mother of a baby-turning-toddler.... And started talking back to her baby, who was clearly trying to talk to us. The mother clearly thought I was crazy, and gave me the same smile and nod I had used on my friend some years before.
People don't look for what they don't expect to be there, and often either lack the skill (or determination?) to really listen hard to what babies and toddlers are saying. My kids have all had very good diction, exceptional for age, but with child #2 & #3 I was listening and talking back from very very young, so I got accustomed to listening hard to somewhat difficult to interpret speech. 10 years later the skill is fading, but I do still find I am much more able to decipher most small children's speech than other adults, parents are often startled and say they're the only one that can understand their child (or didn't get it themselves)... It's such a shame that there isn't an awareness that babies will be trying SO HARD to communicate and if you work hard you can often figure it out.
I guess it is the same root problem as people with disabilities being incorrectly presumed to be incompetent or incapable. We have such a narrow band of acceptable speech and assume anyone that can't meet that standard doesn't have anything to say (that is meaningful or matters).
‘Milestones’ often don’t reflect the full picture. My eldest daughter walked relatively early at ten months whereas my son was relatively late at 14 months but within one week of taking his first proper step was kicking a ball around with good coordination. WRT speech, my eldest spoke several words before her first birthday and was very articulate by eighteen months. On the other hand, no one had any idea what my son was saying until he was three and one day I just happened to catch something he said and realised he was making a very advanced scientific comment with vocabulary that no one would expect from a three year old. Thereafter, I stopped having preconceived ideas about what he might be saying and really tried to listen to the syllables and put them together. If I still couldn’t understand him, I’d ask him to try and explain his thoughts a different way and he would. To everyone else, he seemed to have flourished overnight, but all along was a highly intelligent individual who was frustrated that no one was bothering to make a connection. When he was about eight, he recalled and described to me his boredom and frustration during his early years.
