The really big ones? We could build something large, technologically speaking, but we couldn't boost something like that out of atmo. So we'd have to spend ludicrous resources launching construction equipment to build the construction equipment to build something that large and then boost up all the materials to do so. The cost would be astronomic and then....what? We don't have the propulsion tech to go much of anywhere with it.

If you mean the actual full ship rather than just the size....honestly they're packed with tech we don't have and I'm not sure where to start.

The issue with spacetravel is that A: getting off planets is extremely difficult, and B: distances are extremely long.

All technology that exists right now forces us to burn a high multiple of fuel in your weight to get away from earth (if you weight 100lb then you need about 1500lb of fuel just to get you off the planet, and the same applies for everything you want to take with you, on smaller planets it's a little better though). Escaping gravity is a tough task.

The other issue is that space is huge. To get to Mars with the current in technology we take about 3 months. To get to the nearest other star at the maximum physically possible speed you take 4 years!

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Super simple version is 1) we don’t yet have the tech to travel even through empty space at a rate that would be worthwhile. And 2) we DO have the tech to build massive space ships, but the financial cost is currently too great, with too little reward, for it to be worth doing so.

We don't have enough study on the effect on humans or the levels in deeper space yet to create radiation shielding safe enough to venture far in space, nor the technology to power engines & life support to travel fast enough to make proper space travel feasible (plus the constraints of storing enough fuel, food and other supplies to travel that far). For example, Voyager 2 took 50 years to exit our solar system. Our communications systems are also not yet advanced enough for a mission of such, taking 16 hours to reach or return from Voyager (utterly useless in a true emergency)

Can you evaluate what you mean? I thought ELI5 is FOR explaining semi to complicated questions?

Asking why fictional things aren't real is not a complex topic to be explained. The answer is simple and obvious.

Why can't we build space ships like Star Trek? Because we don't know how. Why don't we build robots like I, Robot? Because we don't know how. Why don't we have cyberware like in Cyberpunk: Edgerunners? Because we don't know how.