Stepping on a landmine doesn't make it explode instead it arms the mine with a noticible click sound then lifting up your foot is the thing that makes it explode.
It has more to do with simplicity of design. It uses the mechanical action. If they had circuitry in there, they could do whatever they want with the arm/trigger process, but circuitry would mean it doesn't stay working for very long when you bury it in mud.
IIRC the whole thing about the land mines exploding when you step off of them is purely down to the Bouncing Betty or the German S-Mine, which saw widespread use and gained its infamy in WW2. They almost worked in the manner described, actually going off with a time delay rather than waiting until the hapless soldier removed his foot from the plunger. But they used a small lift charge to pop the main explosive up into the air a couple of feet and then went off, with the aim of shrapneling in a circle a whole group of soldiers passing by and not just whoever stepped on it. Obviously this wouldn't work so well if someone were standing on it at the time.
The popular conception formed that they went off "after you stepped off of them," which was true in most cases (who was going to just stand there like a nincompoop after you'd just triggered it?) and then Hollywood writers of the era just assumed that most or all landmines worked that way and wouldn't let that misconception go. So now here we are.
Apparently this actually happens, with a very specific type of mine meant for tank infantry. Stupid people just think "some mines work this way, therefore all do."
Kinda like how a decade ago we had the Gluten-Free craze because somehow enough people heard "Some people can't have gluten" and interpreted that as "No one should have gluten"
I've noticed that more and more movies seem to be getting this right. I suspect it's because you can actually create more dramatic tension by showing someone who realizes they just stepped on a mine and now has no options.