So, two things:

1.) Comparing congratulation emails for a promotion and goodbye emails for leaving a job is like comparing apples and oranges. Most people are expected to congratulate a colleague when they get a promotion just as a function of office politics. Whether you get along with the person or not, publicly congratulating them makes you look like a team player and is good optics for when other opportunities come along. On the other hand, when someone leaves a company, there is less social incentive to reach out unless that person has truly had an impact on you and you want them to know. In my eyes, that makes your emails much more meaningful, even if there weren't as many.

2.) As others have said, making friends as an adult can be tough. What I've found to be really helpful is joining groups related to your hobbies. Even if you don't really have any hobbies yet, start thinking about the things that interest you and see if there are others in your area doing those things. It's a great way to meet people and have something on your calendar to look forward to every week. Set your expectations that you're probably not going to make BFF's right away (unless you're lucky or just very outgoing), but over time you'll get to know these people and the relationships will grow naturally from there.

Good luck. You got this. ❤