Why don't construction prices go down?

Not only dehumidifiers, but also various other tools. A wet saw for paving stones is probably also among the things that can be rented out more frequently but are rarely purchased for a 2-day use. Vibratory plate compactor is similar, although they are relatively cheap to buy used.

Basically, your argument ignores the scale and the timing. It makes a big difference whether you usually break even (or can even save for good times) and in an emergency have a loss of 5-10 billion. Or whether you issue special debts of 100 billion here and 200 billion there. Then you quickly reach the scale of an annual budget, which is supposed to materialize out of nowhere through the central bank. At its core, the discussion is nothing more than Keynes reborn, which has already spectacularly failed because in the end, no investment is made in the future or value creation in the country, but ultimately debt is taken on for consumer products from China.
Ultimately, Germany does not suffer from a lack of money in many areas. Much can easily be built by the private sector. Be it wind farms or high-voltage power lines. Here, however, the problem is mainly an excessive bureaucracy and inefficient administration. As a builder, one can sing a song about this on a small scale as well as various companies on a large scale. In politics, however, it is a classic Sunday topic that everyone talks about but no one actually works on.

And what I also learned is that when I have some cash left over, I buy 5 construction dryers and rent them out... Apparently, you can make good money from that

You will definitely make a lot of profit. Every little bit helps. Plus you need a business registration, professional association, chamber of commerce membership, etc. and so on. You are just a theorist.

Spontaneous search on classified ads:

Construction dryers, dehumidifiers with operating hours counter for rent.

Daily rent up to 10 days €5.50 / day / device
from the 11th day €5.00 / day / device

Solar panels or solar stuff in general are becoming cheaper. By now there are 435W panels for €65. I will wait a bit longer, I think the price will drop to €50.

You will certainly make a lot of profit.

The people on classifieds are good Samaritans who work without the intention of making a profit, I had forgotten, sorry

And don't forget… meanwhile there are reporting obligations from the operators, e.g. from classified ads to the tax authorities. I could imagine that some people will be surprised by mail or an unannounced visit.

And don't forget... meanwhile there are reporting obligations from the operators, e.g. from [Kleinanzeigen] to the tax authorities. Some people might be surprised by mail or an unannounced visit, I could imagine.

[Kleinanzeigen] is exempt from this if no purchase is made directly through the platform.

We recently received an offer from Streif Haus for a house (KFW 40 eligible for funding) in expansion stage 3 - FastFertigPlus: with basic sanitary equipment

The price per square meter was between €2,500 and €2,820 depending on the size (150-202 sqm) including some special requests.