I think it depends on where you are from. I'll share a bit of my experience from India:
It seems kind of pointless to combine just a bunch of stock images
I don't think so. It is certainly more than that. I know things like this happen in my small agencies or startups, but most of the time you'll be asked like:
create something unique, don't just make Shutterstock/Freepik design, show some creativity by doing image manipulation etc.
My point is, there's lot more to explore in social media. You don't necessarily have to depend on images and text. You can do typography or illustration based social media posts too. I mean there are many varities of things to do. Of course, photos would be an essential part depending on type of the business, but you'll often need to explore other than that too.
And you can create image manipulation based ads for social media. In my case, I made social media ads around festivals. Like how can you relate a brand or product with a festival. It's just an example. There are many things to do.
Maybe I could just create a good instagram account for myself?
Many people say Instagram is not a portfolio thing. It's just social media. Despite that, you'll see a lot of people using it as portfolio. I know many of my friends especially freelancers use Instagram as portfolio. But you have to spend a lot of time in it. It will totally depend on how you approach it. In my case, I really don't have energy (or I'll say intention) to grow my Instagram for portfolio. So yes, at least in India, if you have some good content and engagement on your Instagram profile, it can benefit you while getting a job or get a freelance job.
Also note that social media is changing rapidly. You might need to invest into videos too to create engagement.
But if you have that much energy and dedication, you'll surely be able to create your portfolio at a better platform. Maybe Behance or maybe your own simple website.
I don't know any recommendation, but you can find many accounts easily online which are curated like a Portfolio. You can go through them.
Note: My intention is not to suggest something like make a portfolio on Instagram. Or create any advertisement or social media post the way I mentioned. I'm just sharing my experience and what can be done.