Modern landmines do have safety features like what you describe.

For example consider this Department of Defence policy from 2020: https://media.defense.gov/2020/Jan/31/2002242359/-1/-1/1/DOD...

“The Department will continue its commitment not to employ persistent landmines. For the purposes of this policy, ‘persistent landmines’ means landmines that do not incorporate self-destruction mechanisms and self-deactivation features. The Department will only employ, develop, produce, or otherwise acquire landmines that are non-persistent, meaning they must possess self destruction mechanisms and self- deactivation features.”

“ For example, all activated landmines, regardless of whether they are remotely delivered or not, will be designed and constructed to self-destruct in 30 days or less after emplacement and will possess a back-up self-deactivation feature. Some landmines, regardless of whether they are remotely delivered or not, will be designed and constructed to self-destruct in shorter periods of time, such as two hours or forty-eight hours.”

This distinguishes “self-destruct” where the mine blows itself up and “self-deactivation” where the mine disarms itself. The first is safer because it doesn’t leave explosive material behind, which could chemicaly detoriate and become unstable decades later. The second is used as a failsafe in case the self-destruct fails.

> Or are the reasons technical

They certainly were when the really old mines were made. Some of them are nothing more than just spring loaded pressure plates. But today modern landmines are much more sophisticated. Some of them can distinguish the seismic signature or a truck from a tank. There are also radio controlled mine fields where soldiers can remotely activate / deactivate the whole mine field as the threat evolves.