Modern concrete is what you need it to be. We know much more about concrete production today than the Romans did. We make compromises, it's not all about quality. If you're building a garage you're fine with it lasting 100+ years, you don't give a shit about 1000+ years. So you don't pay for the stuff that will last for ever. If you're a civil engineer designing a huge dam you go for the good concrete. You also test the concrete before pouring to make sure it's up to spec. Because your budget and priorities make that make sense.