It is amazing how our perspective on our parents change as we move from being children to being adults. If you are fortunate enough to have parents that provided love and stability in your upbringing and seemed to function well as a couple, you may have seen them as almost perfect. It is normal for many children to idealize their parents and their parents’ relationship, especially when you are very young.
Adult relationships change over time and relationships can grow stronger and healthier over the years or they can deteriorate and become less satisfying or less healthy for the people involved. Your parents may have developed new unhappy habits over time (like bickering) or it might be that you just never noticed this part of how they relate to each other before.
It can be especially irritating or sad to be around your parents when you see them relate in ways that seem particularly hurtful or destructive. It sounds like you are bothered enough by this to suggest that they get help. And, in fact, couple counseling can often help couples overcome these patterns.
There is nothing wrong with wanting your parents to have a happier relationship, but you are right; they are adults and it is not within your power to convince them that they could benefit from counseling. While they may not be “perfectly happy with each other” the risk of trying to change things is clearly scarier than the comfort of putting up with their own foibles. Your choices are to make decisions for yourself in relation to their behavior.
Can you see the humor in the situation and not take their bickering too seriously? Do you need to spend less time around them when they are together? You have lots of choices. The only person you can really change, of course, is you.