No, it’s not easy. At all.
I’ve worked in parenteral pharmaceutical manufacturing (IV drugs) and making sterile products at scale is actually quite hard to do.
It is true the ingredients are basic. Where the challenge comes in is running a manufacturing line where all products that come out the other end are guaranteed to be 100% sterile.
And actually calling them “sterile” isn’t true, they need to be “pyrogen-free” or "aseptic". Pyrogens are basically “fever causing” materials like fragments of bacterial cell walls.
You can’t just mix up the ingredients and then sterilize it (heat or radiation). There may not be any viable infectious agents (bacteria or viruses) but it will contain fragments like LPS (lipopolysaccharides) which will make patients very sick.
And the finished product is a fantastic growth medium for bacteria since it doesn't typically contain any preservatives to prevent bacterial growth.
So the process requires a number of steps to remove all pyrogens (micro pore filtration) on a large manufacturing plant which is quite challenging when you’re dealing with ten thousand liter process vessels then filling millions of IV bags. Every vessel, line, filter, connection and package in the plant is a potential source of contamination. You also need extensive testing throughout the process to ensure nothing goes wrong.
It’s one of the reason why there are often shortages of generic IV drugs. It’s expensive to set up and requires expertise to maintain the process. If problems happen, production lines can be shutdown for months while the issue is fixed. As a result a lot of manufacturers just exit the market when prices drop because the profit margin gets eaten up every time an issue arises.
You can read the FDA's guidance document and the cGMP guidance to get a sense as to the complexity.
https://www.regulations.gov/document/FDA-2003-D-0145-0005