The issue is that no one entity should be able to just sit around and dominate the directory of nodes by claiming "I'm a million nodes, wheee!!!", and so that needs to be "expensive". However, for it to be expensive, they also need a way to make money running the nodes.
In our case, people have to lock some money up in a shared pile in order to gain control over percentages of the directory, and what you get in return is that you randomly will get people using you to relay their traffic, for which they will pay you fees (likely close to cost).
(Essentially, instead of Tor's mechanism where, when you want to find a relay node, you randomly select between all entries with equal weight, you would select between market participants linearly weighted by the amount of money they have locked up in the directory.)
And that's really what most of these cryptocurrencies are doing in this generation of new decentralized protocols (which I've seen people hate on): dealing with the reality that there are bad actors and not everyone is going to run the software without evil changes.
This is also the core of Ethereum: you want a decentralized database capable of transactions? OK, well, the order of operations matters (as transactions can preclude the ability for later incompatible ones) and so we need to limit the influence that any one operator has...
...so you build a system where nodes have to spend something--either doing some silly math constantly (proof of work) or locking some money up in a pile (proof of stake)--to gain control over percentages of the directory, and in return they get to charge fees to commit transactions.
(There is additionally often an inflation-based "rewards" component, in the case where not enough people are yet paying fees. I'm honestly not for these and have a way-too-complex-for-here argument for why they might even be "evil" in most cases, but that's kind of irrelevant anyway.)
The reality is that, if you want to build a decentralized system, and you want to figure out how to make it hard for bad people to do bad things, you have to raise the cost of doing said bad things; but, to do that will require programmable money... leading you to crypto.