Various reasons.
First, there's the sheer size of these things.
A carrier of the new Gerald Ford class weighs approximately 100,000 tons and is more than 330 meters long and 40 meters wide (at the waterline).
A draft of 12 meters limits the number of ports that can be used. Not every naval port in Germany is suitable. While Eckernförde harbor is deeper (17.5 meters), where would you even dock them there? Navigation through Danish waters would also be difficult; it's all relatively shallow. The carriers would therefore have to be based in Wilhelmshaven.
Then you need the right shipyard for it. We have that in Germany; the HDW dock of GNY is large enough, 426 meters long and 88 meters wide, in Kiel.
The construction of a support structure is based on fundamentally different principles. Here, it's not about convenience, but about functionality and stability.
Such pictures should not be taken if possible.
The hull is challenging enough; now it's the interior that matters.
First, how will the ship be propelled? For such massive vessels, only a nuclear power plant is really capable of providing the enormous amounts of energy required. Very few countries have experience with this, as a reactor on a ship has completely different requirements than one on land. In the launch vehicle, we want power, and as much of it as possible. It should also be as compact as possible and require minimal maintenance and fuel element replacement. Don't quote me on this, but most launch vehicles use highly enriched uranium. Obtaining this material is difficult and expensive enough.
Of course, a conventional propulsion system could be built, but the enormous fuel consumption practically rules that out.
Now you also need elevators and lots of sensors. That's not necessarily a problem; we manufacture those anyway.
How do you want to launch an airplane? You can either use a catapult, a ramp, or a vertical takeoff and landing aircraft. For the latter two, you have to accept compromises in flight performance.
Then there are other requirements. No carrier is ever alone; the Americans always send a cruiser, at least one destroyer (often two), possibly a few more surface ships, at least one submarine (usually Los Angeles or Virginia class), and a support ship. That costs several billion more.
And then your carrier will also want several squadrons of aircraft. Those also cost several billion.