Ships are only really 'cheap' when measured on the scale at which capsuleers do things.
This is pretty much how I look at it.
If you think about the average capsuleer graduate, after this big investment to find an individual whose nervous system can interface with the capsule and all, they knock around in the starter corp while they are getting accustomed to their new career. Say there are 500 active capsuleers in these corps at any given time, generating 5-10 million ISK in bounties and mission compensation per hour, using all varieties of agents. If the starter corp skims 10% from that income in tax, then they are pulling in, as an arbitrary amount, 0.25-0.5 billion ISK per hour, day-in-day-out, forever*. It's the kind of money that baseliners don't see in a lifetime. All that cash could go to station overhead, personnel, investments, whatever, so long as the capsuleer is invested in the cluster. They do things which move money rapidly and often in large amounts.
As they progress and form their own corporations, they develop their own agendas and acquire assets necessary to the advancement of those agendas, often leveraging them against competitors who respond in kind. They are still invested in the cluster, still moving money from one party to another, but now they reap all the benefit and foot all the responsibility. Maintaining a healthy surplus of assets is a hedge against shortfalls that come with the high stakes of capsuleering in the unfriendly skies.
That's my slap-dash, convoluted way of looking at it.