Am I not seeing something or doing something wrong?
Yes you are doing something wrong; you are changing funds based on recent performance. Don't look at previous performance, ever! This may or may not be a good switch, but you need to understand what these funds are holding, and why a switch is necessary for you in relation your overall portfolio and target allocation etc.
VTIVX is target retirement 2045. Holding 54% US, 36% international and 10% bonds.
VFIAX is the S&P500.
You would be going from being globally diversified (almost 10,000 different stocks I believe), to holding just the top 500 megacorps in america. I this your desired allocation? Do you have other international, why/why not? If you have answers to these and this line up with our plan then yes go ahead. But don't chase performance.
Personally: I don't believe in american exceptionalism so I prefer 30-40% international as well to cover all bases. International has lagged the last decade so if anything it's "due" to outperform (but don't bet on it). The target retirement fund is an excellent, simple 1-fund solution. If you don't have time/desire to learn you could do far worse than putting everything in there for decades then retiring, without having to change a thing! IMO I think you should stick with it, but that's totally up to you.