That’s because of something called the Flores Consent Decree from 1997. It says that unaccompanied children can be held only 20 days. A ruling by the Ninth Circuit extended this 20-day limit to children who come as part of family units. So even if we want to hold a family unit together, we are forbidden from doing so.
The clock ticking on the time the government can hold a child will almost always run out before an asylum claim is settled. The migrant is allowed ten days to seek an attorney, and there may be continuances or other complications.
This creates the choice of either releasing the adults and children together into the country pending the ajudication of the asylum claim, or holding the adults and releasing the children. If the adult is held, HHS places the child with a responsible party in the U.S., ideally a relative (migrants are likely to have family and friends here).
Even if Flores didn’t exist, the government would be very constrained in how many family units it can accommodate.