Companies that care about employees do this. In my view, it all comes down to company culture. The company I worked for, gave me a raise and pen. It was nice. I felt like they liked me and liked me work. I felt like I was now "one of them", because there's a sort of awkward "new guy" period. It's a useful way of including someone into the organization.
Forbes covers this a bit.
- Create A Welcoming Environment
New hires don't yet know the cultural norms at your organization. Hopefully this means they'll share lots of ideas, but sometimes people are quiet until they see how others behave. Make sure new talent knows you welcome input and you're excited to hear their point of view in meetings. To be successful, you have to socialize this within the organization, and over time it will become the new norm. -
Making people feel welcome with a small gesture goes a LONG way to making the company stronger. Not doing it, isn't ideal. So the answer to you question:
Why am I not congratulated on my first work-anniversary with my current employer by my manager?
Is likely the company culture doesn't include this idea OR the manager is busy and will get around to it.