Because it's super easy to buy and hold Bitcoin, if it becomes more popular the value of mining rigs will increase. Right now, there's probably no material difference between buying the coin itself or buying the rig and mining, but eventually, it may become more pronounced.
There are other benefits as well. Buying the mining rig is a sort of hedge in that if bitcoin drops to $0, you at least have some hardware that can be re-purposed/sold.
The more people who mine the less valuable your mining hardware becomes, as the hash rate increase will lead to a difficulty update: it is a lottery that is designed to give payouts at evenly spaced time intervals.
You also now have a slightly old, very special purpose computer... it is like owning a three year old graphics card in a world where you decided graphics cards are not that interesting to own... what exactly are you "repurposing" it for? If it is some really awesome ASIC-based mining rig it might only be useful to mine Bitcoin.
But the more popular the coin becomes the harder it becomes to mine it. If the increase in mining difficulty (driven by total network hashrate) increases faster than the price of bitcoin then your mining apparatus becomes less and less profitable.
As for repurposing hardware, that might work with GPUs (although their value depreciates pretty quickly) but the Antminer is a custom ASIC, so it can't do a lot more than bruteforce SHA-256.
I think it depends on whether you think the price will go up, or merely remain high. Mining lets you make money even if the price remains stable, as long as it's already sufficiently high. To make money with buy and hold, the price needs to increase.
There are other benefits as well. Buying the mining rig is a sort of hedge in that if bitcoin drops to $0, you at least have some hardware that can be re-purposed/sold.