You'd think a step would be pooling resources to mine. Like people invest in super computer time and share rewards. Maybe the value of BTC is too low for that now? Don't universities and governments still have super computers?
Err yes, but both Unis and governments have super computers for reasons, and that reason isn't mining bitcoins.
There is "cloud mining", which is close to what you are talking about.
In the end, it comes down to what is the most efficient way to mine bitcoins, and that is ASICs to date, something else in the future, but it always be something dedicated to the job from now on.
And sure, you can use "spare computing power" from any device, but that computer / electricity is always owned by someone or some entity, and you are probably breaking laws / terms of contracts if you spend their resources on bitcoin mining.