That's public transport for you. You pay for one ticket, you get one seat; you don't get a whole row to yourself just because you don't like other people.
Pay for first class if you want more space and a seat on your own.
You're getting downvoted, but as a woman I have been and have seen other women hemmed in by creepy men for sitting in the window seat. I'd rather be rude than at risk.
Sometimes dangerous. I once moved when someone at the station smoked close to me, and he took it as offense and on the train he sat across the aisle from me and stared at me the whole ride until he luckily got off 2 or 3 stations earlier than me. I just kept pretending to read in my ebook reader while watching out of the corner of my eye and plotting my escape should he get off at the same station as me.