Because anyone who's been following this debate for a while is well aware that 'assigned' is appropriated language from the time before clear chromosomal tests for people with DSD were available, when sexing a child with DSD could be a matter of 'best guess' and this is an area of other people's hardships that I'm not going to go further into because people with DSD have repeatedly asked not to be weaponised in this way by those with unambiguous biological sex who would like to be able to blur the boundaries for their own purposes.
It is helpful in the TQ+ political debate to encourage the use of 'assigned' which implies that gender is something coercively imposed at birth that the person may later reject. However it attempts to conflate sex with gender and make it so that biological sex escapes boundaries and definitions and can be self defined and by doing so tries to remove everyone else's boundaries and sex based rights.
In short: a baby's sex is observed or established by actual, fixed, unchanging fact, and it is not 'generally' male or female, it is one of two sexes. The child's gender is something they will tell you about when they choose to. Probably in a very great amount of detail.