Or in the case of batteries: Requires a whole lot of hardware to be bought to get a reasonable economy of scale.

True and batteries have their own characteristics.

They do not last forever, have inefficiency/loss (order(s) of magnitude larger than slippage/fees) etc.

It can seem like the Wild West with many opportunities for arbitrage, but there are practical challenges.

Many countries prefer to improve energy infrastructure through private initiatives, and this is one of the incentives as I see it:

If you get a foothold, profitable trades can be made with only limited competition.

This should also help decrease volatility of electricity-prices, as predictable fluctuations gets evened out from arbitrageurs.