Since the customer must always be put first, why is it that most organisation charts put the CEO/Chairman/MD on top?

Customer first does not mean that you put him on top of your organisation chart, It means that their needs to be clearly diagnosed and their objectives understood prior to any service being delivered to them.
To be given access to the full range of services available - whether delivered directly by you or by another more appropriate service provider.
To be clear about what they can expect from any service you deliver to them and the timescales within which that service will be provided.
That you will continually review their needs and follow up after services have been delivered to them to have open access to your customer feedback and complaints procedure and that their feedback on services will be listened to and, where appropriate, will inform the development of future services.
Services to be clearly targeted, relevant and appropriate for them.
Our people to be appropriately trained and developed to have the necessary skills to diagnose and meet the customers' needs.
Our people to deliver an impartial and objective service operating in the best interests of customers at all times.

CEO, Chairman or Md are on top of the organisation's leadership, Administrative & Management hierarcy while customers are the top of the organisation's priorities & strategies (customer forcused or customer centric) thus organisations have to put customers are the forefront because they are the biggest asset an organisation can have.