I see two main reasons for this.
For one, different models of notebooks and phones vary widely in their electrical specifications. Each one uses different battery models, which in turn provide different voltages/currents/power, which in turn require different electrical characteristics on the chargers they use.
Batteries are different basically because each model has different power specification, characteristics and devices, and also evolving battery technologies call for different chargers. For example, notebooks use much bigger batteries, and the CPUs are optimized for computing power while phones are meant to save energy at the cost of CPU power, hence they use smaller, more portable batteries.
And in turn, each one ends up using a different voltage and currents, requiring another charger.
It's theoretically possible to put adapters in front of batteries so that a single charger satisfies all, but that raises the cost of every computer out there and increases volume and weight, something that few people would accept. Variable chargers are also possible, but that also makes it more expensive for little reason. This is the part where it's practically difficult to make such a charger.
Another reason for not making such chargers is the vendor lock-in
it creates. This is obviously bad for the user but highly desirable for companies making the chargers, as it forces the user to buy a specific model from a specific company, with little chances of finding an alternative supplier or a better model. This is the part where they "don't care"