Because they think it's what you want to hear
Because they think it's what they will do
Because they think it's polite to say so
In the meantime a million and one things happen, many men don't want a regular girl, or are passing through, or decide it/you aren't for them, or realise their finances aren't up to it, or go on a guilt trip, or simply don't want to.
No customer is obliged to return. If they do great, if not as long as you given a good service, got along well I don't see any problem.