It says on this page [How to buy Stellar Lumens (XLM) in the US].
If you live in the United States, you won’t find a cryptocurrency exchange offering Stellar Lumens for direct purchase.
You have to first purchase BTC or ETH and only then can you exchange that for XLM.
Why is that? Are there any pros/cons of allowing someone to purchase using (say) USD.
Rather than making them go through the process of purchasing other crypto currencies first and then trading them in for stellar tokens.