There are two major things.
1. I don't suppose you've had to finance your own lodgings and living expenses. If you perform the calculation, a modest lifestyle can easily cost £15,000.00/year post-taxes. If you're a full time worker earning £7.00/hour, you're only earning £14,500.00 or thereabouts gross-income. That leaves no room for holidays, emergency expenses, savings for a pension or mortgage, income-taxes, etc.
There's high anxiety and low economic mobility - in effect, wage slavery. The economic argument makes a lot of sense when someone has a lot of debt, or has a specific goal like a downpayment for a mortgage, or wants to live a better lifestyle.
2. It seems you're under the presumption that there is no choice or screening or selection process, and the escort is obliged to accept all clients. This is not the case, escorts may choose to reject any clients she wishes, and has a lot of freedom with respect to what services she's willing to offer and what safety precautions she chooses to take (insofar as paying for a bodyguard in some instances).
She might choose not to discriminate based solely on physical attractiveness, because that would be like a waiter only serving attractive patrons, but she may exclude certain age ranges or backgrounds as it does involve consent. She may choose to only see hygienic, respectful patrons who don't chain smoke or have a 50 inch beer gut. She may choose to not provide services that physically hurt her, or otherwise make her feel uncomfortable.
It'll be quite a different discussion if you're talking about street prostitutes, where arguments about exploitation (involving some sort of implicit/explicit coercion) comes into play, as well as all the other risks like assault/battery/drugs/trafficking/STDs/etc are much more prominent.
Perhaps you don't have the financial pressures of these people, and sex is more personal to you. It's not true for everyone, and escorts can lead perfectly happy lives.