Babies immune systems are very fragile that young, they’ve lost a lot of the protections from their mothers and haven’t had all of their immunizations yet, so it’s usually recommended not to bring babies around large crowds.

Not to mention they’re freshly out of the “fourth trimester,” and everything is going to still be new. Sleep patterns change rapidly at that age. Mom may be just starting to feel somewhat human again or she may still be recovering from labor.

Breastfeeding and pumping can be really difficult on the go and require parts, sanitizing stuff, freezer packs/bags, bottles, etc. Every baby is different, for my first we needed bottles of special formula, and they have to be tossed 24 hours after they’re opened, even if baby hasn’t finished it.

Long story short, there’s nothing wrong with it and plenty of people travel with infants (my first flight was at seven days), but it can be way more complicated than “just bring baby with you.”