I understand the sentiment. But before I did an internship, I was also in the same position as you where I had been coding a lot. However, like others have mentioned, an internship is very helpful for the following reasons:

1. You get experience working with important concepts that exist in production like proper git usage, technologies like Kubernetes and Docker, managing differnt build flavors on the client, unit tests, how to review other peoples code professionally, etc

2. You learn the soft skills part about building software. More often than not, software is a team effort and learning how to navigate different peoples egos and personalities and trying to get your ideas across are very important. Even if you're the best coder in the world, if you can't get your ideas across, it doesn't matter

3. 3 months is actually a lot of time to get meaningful work done. Even full-time employees will have feature development that takes around 3-4 months from ideation to shipping (in high velocity companies). As an undergrad, you have 3 summers and I would consider at least allocationg 1 of those summers to an internship to see how you like it