All pediatricians? Not so!
It is recommended by:
World Health Organization
UNICEF
US Department of Health & Human Services
American Academy of Pediatrics
American Academy of Family Physicians
American Dietetic Association
Australian National Health and Medical Research Council
Royal Australian College of General Practitioners
Health Canada
that it's best to wait at least 6 months before starting solids because it gives the baby's digestive system time to mature, gives them a greater protection from illness, decreases the risk of food allergies, helps to protect them from iron-deficiency anemia, helps maintain your milk supply, protects from future obesity, and it makes starting solids easier because their tongue trust reflex should be gone.
Most pediatricians are not well versed in infant nutrition. It's none of their business if you don't take their advice and follow what you feel comfortable with as long as you are not putting baby in any danger.
Breastfed babies can thrive perfectly with delayed solids!!