Because when the Apollo project was ongoing, they only built what they needed to build. Everything was a prototype basically and there were usually different versions of everything going around. Afterwards a lot of the stuff was re-used for later programs, often modified or taken apart for parts. As the budget shrank they needed to be creative. Take a look at the work CuriousMarc and his team is doing with repairing and restoring old Apollo Moon hardware, along with documentation and preservation.

Why can't we simply build the Apollo lander today. Well a couple of reasons.

First of all, like I said it were prototypes, so you'd have to figure out what design to use. All of the documents back then were on paper and not all of it is digitized by a long shot. The amount of documents they produced back then was crazy. And a lot of it was lost over time unfortunately. Puzzling all of that together would be quite some task. Most folk from back then are since dead or at the very least retired. And I for one sometimes forget entire projects I worked on, so good luck getting small details out of those people.

Our idea of what is acceptable, a good idea and safe has changed since the Apollo times. A lot of the design back then included components that were very dangerous and toxic. Not only to be used, but also to manufacture, which we wouldn't find acceptable these days. And things we've later learned were a bad thing to do. So the design would need to be modified to be safer, which would probably cascade into an entire new design.

We've lost so much of the support infrastructure the program relied on. It's hard to understate how much this matters. This is a big thing when people say the moon program was fake. It wasn't just one rocket, one lander, one crew, it was millions upon millions of pieces of infrastructure supporting the whole thing. From jigs to electronics, test equipment, custom tools, handling facilities etc. All with their own backstory, design requirements, documentation etc. A lot of this has been lost, especially when it was outsourced at the time. You'd have to reverse engineer and re-create a lot of that.

Time has moved on and so has technology. Whilst the Apollo program had some cutting edge stuff back then, these days it's ridiculously outdated. It would be very hard to manufacture any of those components today. We're talking about the first generation of integrated circuits, on very expensive ceramics. Using crazy analogue electronics, only understood by the best gurus at the time. Even mechanical computers were used, a lost artform last used in the 80s. You could start redesigning stuff to modern equivalents, but again that would probably snowball into just designing a whole new thing.

Recreating something from that long ago is simply not possible I'm afraid. And even if we could, it would probably make for a pretty shitty lander compared to modern standards.