Because static electricity is generated whenever objects touch, whenever wind touches an object, etc.

Normally some objects can dissipate the charge if they are conductive (for example a tree).

But many man made objects do not conduct at all or are isolated from ground (for example car is isolated through non-conductive tires).

For these objects, humid air helps slowly dissipate the charge, but when humidity is below 30% this dissipation becomes very slow process and a large amounts of charge can accumulate.

I have home electronics lab and I keep a humidifier and ionizer to provide humidity and ions to dissipate any charges that can accumulate on any objects (like me).