They may think that whatever their true reasons are will be dismissed out of hand or alienate you.
For example, they may think that you guys are still developing as people and adults and be worried that if you had kids now, you might grow apart in about 5 years or so once you've come into your own as an adult more. You could be great and compatible with each other now, but in 5 years be horrible together. It happens at your age, changing a lot in your early twenties is just part of being in your early twenties. Even if you try to grow together, that doesn't guarantee you will. Then there would be kids involved in a nasty divorce. This happens a lot with young couples. If you say it to them they will all say it won't happen to them....yet it happens a lot anyway. So many young marriages don't really have staying power. They want to see for themselves that yours actually does have that staying power before children are brought into the picture.
They could also be concerned that you aren't mature enough and independent enough for children. As a parent you are going to need confidence in your own opinions, choices, and abilities. The very fact that you think your parents can forbid you from this shows you haven't developed enough of that confidence in your own choices that parenthood will require from you.
They also may not feel confident enough in your long term independence and economic stability yet. It wasn't long ago that you were likely somehow dependent on them - economically or for school. They may be afraid that even if you are stable right now, the extreme change in finances that having a child will create may be more than they feel you can handle, and they don't feel up to being your fall back. And since your relationship hasn't proven it can last long-term yet, they may be worried you will end up a single parent and then expect them to pick up the slack. Even if you do stay together, they may not be ready themselves for demands like free dare care, having you move back in if something happens with your housing situation, etc. that can happen either if you stay married or if you become a single parent. They may just be feeling great about no longer having responsibilities with kids in the house anymore, and are not ready for you to basically start round 2 of their being involved in kid's lives again.
They also may feel that you haven't built enough of a foundation in your life yet. You may be doing well in your careers, but could go further and prove you are stable in the long haul more. You could get Master degrees or further schooling, work up the corporate ladder while it can be your main focus, and in other ways create a better economic foundation for your life that having a baby instead may make much harder to do. They may also be especially aware, now that you are out of the house, of how much they missed being able to just be a couple together and be able to travel and do other things they like without worrying about a kid's demands. They likely missed that for 18 plus years, and not be willing to give that up to fulfill the role of Grandparents. They are also especially aware of the value of having time as a couple, and want you to experience that before you throw away that chance. It could be decades before you have that chance again.