Because they are more dignified. Usually. Meet one of them.
I refuse to name my regal noble German Shepherds “Spot”, “Blackie” or “Tootles”.
Although I also reserve the right to call them by silly made-up “pet” names. ;-D
LOL
“Biff”
“Fluffy”
“Twinkie”
That's the only way to go for me. I think giving animals human names devalues humans.