The reason is much more Machiavellian, namely if you keep the price of entry high and you loose money to mine for 80% of the time then the 20% of the time that it's extremely profitable you have a bigger multiple since the competition is weaker then it would've been otherwise.

is mining competitive, though?

Absolutely. A fixed reward (25 BTC) is rewarded each time a block is mined. Each block is based on the prior blocks in the chain, so when one miner mines a block, all the other miners must start over. Blocks are mined on average once every ten minutes, so basically, a random miner is selected approximately once every ten minutes to receive a reward.

Saying they have to start over is implying that you're somehow making progress. The only progress your are making is just ruling out a few inputs from a massive search space. You don't really lose any work when it resets because the work you've done is useless.

I understand the semantic difference, but I'm not sure the distinction matters in any way. The implication that there is progress is tenuous at best.

Of course they lose work. They have to pay the set-up cost of bundling a new set of transactions for the next block.