Working with multiple tons of material that dries out as you move it around is hard. There are a lot of steps between the concrete being mixed and when it finally reaches the pour.

Cutting out a piece of a slab and sending it to a lab is for post-pour validation in serious construction. There are pre-pour tests that are much simpler depending on the seriousness of what you’re building.

The slump test is rather simple, for example: https://en.wikipedia.org/wiki/Concrete_slump_test

It’s basically a cone with handles and a procedure that’s easy to learn.

No - it's actually local variance in materials coupled with the difficulty in moving materials between markets economically. Some areas just have better suited limestone or gravel or sand and can afford to build resilient structures for a fraction of the price that it'd cost in other areas.

This issue here is mainly that it's very expensive to ship all the components of a Concrete in the volume necessary in an economical manner. Some areas of the world just lost the lottery when it comes to having resilient building materials.

Corruption absolutely is an issue as well - I don't mean to downplay it - but even if we remove it as a factor there are just a lot of variables involved in making a reliable Concrete... finding a good mix is an artform and if, for instance, your limestone quary suddenly hits a more clay-laden amalgamation then your Concrete that was reliably lasting for three decades under certain conditions might suddenly lose a decade off the expected lifetime. That change in material quality can also be difficult to detect so there are real quality assurance issues in Concrete mixtures outside of just corruption and cutting corners.

They are standardized for a given mix. A mix design that is based on a trial badge is submitted to the SEOR prior to pouring anything. The mix design shows the ratios ingredients (cementitious materials, find and coarse aggregates, water, air, admixtures). But Concrete is still a non-homogeneous material with lots of variations. Take for instance aggregates, if it rained the last two weeks, the moisture content will be higher but it may only be a layer on that pile. Same goes for gradation (particle size of the rock). Sometimes you get a batch with smaller rock. There are a 100 things that can go wrong to get bad mud.

But yeah, there are concrete plants that cut corners and try to save on cement (the most expensive part of the mix), which depending on the project may bite them in the ass when they have to pay to fixing it.

My grandpa used to be a concrete inspector (for the state of Minnesota - if you ever drive i394 there he was one of the inspectors for that). Different plants within a normal commute of his house often had very different sand and so needed a different mix.

When you're making tons of something process variations get magnified.