Nice photo!
Remember that the US wasn't always as powerful or rich as it is today. Remember that making large memorials and glorifying someone's legacy hasn't been part of US culture than for more than a few decades... most coins, for example, just had a general design for over a century, using it to honor former presidents is just something of the last decades, really.
So that would explain why this grave may be relatively humble. But then, it's still there. That's remarkable in itself. Most of the graves of my ancestors have already disappeared, the furthest back I can go is my great-grandparents and they died in the 20th century...