Cause and effect...
Cause there is oil dripping on the starter......
It's not supposed to be oiled, only operates (hopefully) for a few seconds.
I won't bore you with 'my '97 Super Duty has 600k+ miles and only replaced starter once'. True story.
Effect...
Oil seepage disrupts the electrical current, dirt and crude gathers on the oily residue making matters worse. Inside and out.
To put it in perspective, take a scrap piece of metal, any metal, place a couple drops in the centre, stick it in a corner, on a shelf, under your car, someplace out of the sun and out of your way. You will be surprised how much it spreads in a week or so. Imagine months and miles, heat cycles, you get the picture.
Replace the starter pay'n attention to the condition of cables and connections, fix the oil leak, most times it's the valve cover gaskets.
Now go forth my friend and be powerful