why is not possible to create my own route?

Routing on OpenStreetMap is not driven by AI, it's just a straightforward algorithm. If it doesn't give you the fastest route then either there's an error in the map or in the routing engine. Some web sites allow the setting if intermediate points, e.g. routing.openstreetmap.de where you can simply grab and drag the route.

Hello, six months ago I started to travel a lot on OsmAnd navigator.

First of all, I want to say that the navigator calculates the best route based on the data that it has.
Secondly, perhaps your navigator does not take into account traffic jams, that is, it plots a route without taking traffic jams into account.
Thirdly, if there is time, knowledge of the road and desire, check the correctness and completeness of the data in the OSM, set speed limits, traffic lights, pedestrian crossings, turn bans, and so on (of course, you need to place real objects), wait until the navigator updates the data from byza.

If none of this helped, then only adding intermediate points when laying the route will help.

I added a lot of objects (really existing ones) along the roads that I know well and drive on them. After detailed mapping, it is possible to "slow down" the traffic to such an extent that the navigator shows the correct result. But this will only help in areas where there is almost no traffic jams. If your navigator does not take them into account, then this is unlikely to help, you need to help the navigator get the right route before each trip.

You maybe be able to find the reason for strange routing. If you move the start and end points, the red and green markers, around you should be able to locate the reason. That will be where the map will need editing ... maybe. https://www.openstreetmap.org/directions?engine=fossgis_osrm_car&route=50.812%2C19.113%3B51.109%2C17.033#map=7/50.563/18.428 Warning. Once the problem is corrected we have to wait a few days for the routing engine to start using the new data.

I'm looking for something similar, but I want to plan the route so I can make it a scenic motorcycle ride (no highways). Is my answer to manually put in way points? Or can I use routing.openstreetmap.de to save and share routes for embedding on a web page? The latter looks, so far, like it works.