When you source the file, the assignments will be set but the variables are not exported unless the allexport option has been set. If you want all the variables to be exported, it is much simpler to use allexport and source the file than it is to read the file and use export explicitly. In other words, you should do:

set -a
. file.txt

(I prefer . because it is more portable than source, but source works just fine in bash.)

Note that exporting a variable does not make it an environment variable. It just makes it an environment variable in any subshell.

source (.) vs export (and also some file lock [flock] stuff at the end):

In short:

1. source some_script.sh, or the POSIX-compliant equivalent, . some_script.sh, brings variables in from other scripts, while
2. export my_var="something" pushes variables out to other scripts/processes which are called/started from the current script/process.

Using source some_script.sh or . some_script.sh in a Linux shell script is kind of like using import some_module in Python, or #include <some_header_file.h> in C or C++. It brings variables in from the script being sourced.

Using export some_var="something" is kind of like setting that variable locally, so it is available for the rest of the current script or process, and then also passing it in to any and all sub-scripts or processes you may call from this point onward.