Unfortunately a son is a son til he takes him a wife, a daughter is a daughter for all of her life (as the saying goes).
Women are (generally) very close to their mothers - their mothers raised them, wiped their tears, were their biggest cheerleaders and there is a lifetime of memories and trust there that is a completely different relationship to the one she will have with a mother-in-law who didn't appear in her life until adulthood. Therefore it makes a lot of sense that a woman would naturally navigate towards the most trusted woman in her life whilst trying to navigate parenthood (same as I'm sure your son would go to his dad before DIL's dad). When she visits her mother; it's not just about grandparent access to the grandchild, it's about mother and daughter spending time together.
On top of this, I'm sure you remember that when a baby comes along, that maternal instinct kicks in. You feel super cautious with everyone around your baby (it's a completely natural and normal instinct) and leaving your baby can come with all levels of anxiety and worry, so again it makes sense that she would want the most trusted woman in her life to watch her child above others.
I understand your son is a parent to this child too but she's the one that carried the baby for 9 months and went through all the physical and hormonal changes, she was the one who went depleted of energy as the baby took the nutrients they needed from her body, she's the one who put her life on the line to give birth and then had to deal with all the physical and hormonal issues afterwards ... meanwhile your son was a spectator. On top of that, when children are here, it's normally the woman who takes the main childcare role so she will do whatever feels comfortable for her.
The best thing you can do is firstly, stop the comparisons and tally chart re who gets more time with the grandchild. This is a baby, not a sports match - things don't have to be 'fair' or 'equal'
It's disappointing to hear but it's the natural way that things are - all you can do is be kind and supportive to your DIL. Don't be overbearing or push for more frequent visits, offer help where you can (not just with the baby) and make her feel secure, You could tell her that you would be happy to babysit if ever she would like you to but also say that if she feels she's not ready for that yet then you completely understand. If she does ask you to babysit, ask about baby's schedule and if there's anything else you need to follow while the baby is with you (she needs to know any boundaries will be respected)
It's hard not to feel disappointed but she may come round if you approach it in the right way.