Because the B visa is not to be used for actually living in the US. CBP will rightly wonder how she can afford to have such long vacations in the US, and whether she is working here remotely, which is not permitted.
If she wants to spend so much time here, she needs a green card.
Also- you seem to think that the 6 months is an entitlement . It is not. CBP can admit her for any length of time under 6 months, or they can deny her entry if they suspect immigrant intent, which is very likely as she appears to have no job or other ties to home, as evidenced by long trips to the US.