How to concatenate strings containing asterisks in bash

I have 2 variables:

file1=file1_*.txt
file2=file2_*.txt

The actual names of the files are: file1_FY24Q3.txt and file2_FY24Q3.txt and they change every quarter. I want to concatenate these vars into one string. I tried this:

files="${file1},${file2}"

But this doesn’t work and echo $files prints file1_*.txt,file2_*.txt.
Why doesn’t it work and what is the correct way to concatenate the strings?