The problem is that every function has access to them and the large a project grows, the harder it gets to keep track of what is doing what with what and who is depending on which variable and who is writing into which...
To give you a hands on example, let me quote yourself:
ground_contact, wie ich aber gerade feststellen musste, ist keine variable, die nur 0 oder 1 annimmt, sondern die zähle ich bis 16 hoch.... weil ich irgendwo irgendwann vor jahren mal die dauer des bodenkontaktes brauchte
English (translation by me):
ground_contact is, as I just discovered, not a variable in the range of 0 to 1 but is incremented up to 16 because years ago I needed a the time of the ground contact
That basically sums up why global variables are considered bad practice. Class dismissed
