(I haven't seen the movie yet)
I think you're right to ask for some answers to those kinds of questions. We're sick of seeing other superhero origins because we're already familiar with those ones. No one's going into Batman not knowing his origin. But a lot of people haven't even heard of Shang Chi, so a bit of background would help.
Cool thing about movies is there are a ton of ways to get information across as well. We can see parts of it in a flashback, but things like staging and character reactions/emotions can convey a lot as well. It still blows my mind how they've got thousands of people working on these movies nowadays, yet they underutilize every tool at their disposal.
I mean, is it really that hard to write a story that doesn't have 80 plot holes?