Laptops are completely commoditized, of course you would struggle with profitability there. For a company the size of Valve, building these devices probably carries very little risk. The biggest risk is the software, which they were going to tackle anyway.
I can tell you from personal experience this isn't easy at all and is just as difficult as any other hardware project, even if you're not literally building new processors or hardware firmwares it's still a huge mission getting something like that to market let alone making it profitable.
I work on a company that has shipped laptops where we designed all the casing with off the shelf processor/storage/etc, so have worked on what you're describing here.