Why do I have to set LD_LIBRARY_PATH before running a program, even though I already linked the library locations in the compile stage?

I am compiling a model using make. The model has a Makefile that connects the source code with dependent libraries via flags that look like -L/lib1 -L/lib2. But when I try to run that model, it fails unless I also ensure the environmental variable

export LD_LIBRARY_PATH=/lib1:/lib2

and points to the exact same libraries. This seems redundant to me. What could be going on under the hood here? Why do I effectively have to specify the location of the libraries before compilation and before execution? This might be a silly question; I'm not very experienced compiling to machine code, usually just use scripting languages.