Sure, but a cleaner coming twice is the same cost of a robot vacuum that will work for a couple of years, typically. They do an okay enough job, but they need to run daily, sometimes twice a day, to really keep up considering it's limitations.
Robovacuums don't cost £150 an hour. If you buy one for £500 and run it every day for two years, you're paying ~70p per hour. Are there any cleaners who charge less than £1 per visit?
I used to pay my Spainish cleaners about €20 euros a week for two cleaners. Granted that was while ago, but it was peanuts.
Also I'd rather have cleaner do it properly, than by a robovac that (as everyone says on the sibling comments) does half a job.