Garbage collection makes the performance of the code much less deterministic.
A lot of embedded loops running on embedded in-order cpus without an operating system use cycle count as a timing mechanism etc.
There exist options between no reclaim and using a garbage collector which could be considered, depending on the exact technical specifications of the hardware it was running on and the era in which it happened.
But retrofitting technical reasoning about why this may have been done is superfluous. The contractor already said why they did it, and the subtext of the original post is that it was flippant and hilarious.
"Cared only enough" is just your projection. The contractor knew the requirements, and satified the requirements with no waste of engineering time, and no risk of memory reclamation interfering with correct operation. The person complaining about leaks wasted both his time and the contractor's.
When your job is performing an analysis of the code, five minutes asking for a dangerous feature to be justified is ridiculously far from a "waste of time".