> am kind of still confused why the industry links „keyboard controllers“ with „synths" there should be just good keyboard controllers that can plug into good synths (like Eurorack). What am I missing?

There are good keyboard controllers that you can plug into a Eurorack if you're a Eurorack player. Many synths haverack mount versions. You're missing nothing in that sense.

Your question isn't about the industry. It's about musicians. You're asking why musicians other than you still want complete instruments.

The answer is simple. People who play other instruments need other instruments. Of which the keyboard can be an integral part.

You might as well ask why people still buy hardware synths when perfectly good software synthesizers exist.

The answer is the same: they play differently (that applies both to the machines and the performers).

Here are some reasons why synths come with built-in keyboards.

The most important reason:

— Keyboards are mechanical instruments with particular response curves. The patches the synth comes with can be optimized to play and sound good on the keyboard the synth ships with.

This guarantees quality out-of-the-box, consistency, and makes it so that your presets would feel the same when your play them on another device.

Yamaha DX-7 played with an 88 weighted keys keyboard is a different, and a worse instrument. As is a Clavinova played when DX-7 as a controller.

— Ergonomics matter. Each synthesizer is unique in the way it builds its sounds, which is why many come with dedicated controls (buttons, knobs, sliders, screens).

Where these controls are in relation to the keyboard matters.

I can play drum pads while playing the keys on my Akai Mini Play 3, for example, but not on my larger Yamahas. I can tweak the patches on the Reface DX live, but not if I used the MX-88 as a controller.

— Physical layout matters. Performers to look the audience, not the knobs; the audience wants the performer to look at them.

Memorizing the layout by touch isn't possible when the layout isn't fixed.

For gigging musicians, the following are important:

— portability

— physical robustness

— time to set up on stage

— space on stage

— physical setup

— ability to move while performing

Having more cables to connect isn't something gigging musicians want when you have 5 minutes to set up for a 30 minute set at a dimly lit bar.

And then there's the setup. You put a stand on a stage, there goes your controller. Where does the synth go? Oh, you need another table or stand for the synth.

Now to tweak a patch. Your controller may not have all the knobs your synth engine uses. The sound module has them, but it's on the table over there, while you're playing facing the audience over here. Bummer.

Finally, I have straps for some of my keyboards, so I can move around the stage during the show.

1/4" cables are long, and made to withstand abuse; USB plugs agent, and MIDI cables, while better, aren't made for that either.

There are also marketing reasons:

— A synth that people can try in a store is more appealing than one they can't. Rackmount synths don't make for good displays.

If people have inconsistent experience trying the instrument, it doesn't bode well for the brand.

— When someone plays a Nord keyboard, you know it, because it has a distinctive look. Musicians take note of other musicians' gear.

— An instrument that can't make any sound on its own is not an instrument, it's a module.

It's a different product.

And then we get to psychological reasons:

— Performers are deeply attached to their instruments.

A sound module without a keyboard isn't an instrument. A controller isn't an instrument. Together, they form a system made up of components; it doesn't feel as an instrument the same way a guitar does.

— Looks matter. Performers want to look good on stage. And a generic controller simply doesn't look as good as a complete instrument, with a dedicated interface.

— Feels matter. The feedback one gets from touching and playing the instrument affects the performance.

And that's before we get into things like:

— busking

— jamming with other people

— playing outdoors (I played my Reface DX in the mountains and at campfires, in the streets and on the road)

The TL;DR, though, is that if you want a controller for your Eurorack, you've got it.

Saying that's how everyone should make music, though, is quite a reach.