Not all eggs are good enough for fertilisation, even if you're an 18 year old in perfect reproductive health.
Not all sperm are good enough for fertilisation, even if you're an 18 year old in perfect reproductive health.
Plus, there's a much smaller window of opportunity for sperm to meet egg than most people realise. Theoretically, the egg goes off the boil (as it were) a few hours after ovulation. Sperm can hang around for a good while, but again it won't necessarily be in great shape.
You could easily miss ovulation, even if you're using opks or charting or symptom spotting - none of these are fool proof. Yes, very regular sex will minimise the chance of missing it, but this is hard for most couples to do month after month.
Plus, women tend to have anovulatory cycles occasionally, even if they don't have fertility problems.
Even if conception occurs, there's a good chance that the resulting embryo will have a chance defect that may mean that it fails to even get to the implantation stage.
Honestly, when you realise all the ducks that have to be lined up in a row for a baby to result it's a wonder that there are any at all!
Ds1 took us 2 months to conceive. Ds2 took 3 years, but if I'm absolutely honest, out of the 40 or so potential cycles about half of them we probably had no chance as we hadn't dtd at the right time for whatever reason, a couple of times I had some odd symptoms and may have had a very, very early chemical pregnancy (but no bfp so could have just been one of those things), factor in our age and some bad luck and you have 3 years. But there's no real reason it couldn't have taken us 1 month, or 6 or 12 or 24. Nothing had changed between the first cycle and the successful one, it was all a roll of the reproductive dice.