First off it is VERY obvious you don't have children because an 11 month old is still very much a baby in every single way, from the crying to the wearing onesies, nappies and requiring regular milk, and including officially according to the NHS.
In the toddler years, they start seeming more grown up in many ways, walking, talking, etc., but they are still very much babies in many other ways too, behaviourally, physically, developmentally...
Then there's the fact that as a parent you often genuinely don't notice how much they have grown, because you see them every single day.
But then, just generally, I think a lot of people just say it because no matter how old they get, their parents will still always remember their children as babies and love them just the same.
I met a man once whose family still called him "Baby David" well into his 40s... I thought it was quite sweet