There's certainly no harm in running daytime lights.
But most people are lazy and won't do anything that they're not absolutely required to do. Take seatbelts for example.
It takes effort to buy bike lights. To attach them. To remember to recharge the batteries. To remember to turn them on. It's just a little extra effort and most people don't want to invest that little bit of extra time and energy beyond what's absolutely necessary.
On top of that, 90% of bike accessories strike me as cheesy and esthetically unpleasing.
I have 4 taillights.
One is attached to the seat post purely as a backup.
I have a helmet taillight which is always on at night, and sometimes on in the day.
A third is attached to the rack, and the batteries are close to done.
A fourth is a $1 special from china bay. It works ok. Its attached to the rack bag, flopping around and tilted at various odd angles as the light bounces around during a ride.
None of it looks especially nice, but the helmet tail light works reasonably well enough on it's own. But it's nice to have the rest as a backup. Plus, I'm too lazy to change the batteries on the rack light until the batteries are down to zero.