1 - I don't believe there has been a job to take for around 3/4 series
2- They aren't 'Hired' at the start of the process

Being on a show is a job.

It's part of the format. It doesn't have to make sense.

He could say "you're eliminated" but that's got rather sinister overtones.

It's because that's what Donald Trump says on the US version, and the BBC demand that Alan Sugar says it. He's completely open about the fact that he hates saying it because it's an absurd thing to say, but I suppose he's quite passionate about The Apprentice generally, and it's a small price to pay.

It's the shows catchphrase, we see him saying it about 10 times an episode from previews and such. It's just a good marketing tool for the show.

When you see a Tshirt saying "Your Fired" on it, you think of the apprentice etc.

As others have said, it's the show's format now. It shouldn't be called "The Apprentice" any more either but "The Business Partner" isn't as snappy!

The show is all about the process the candidates are undergoing in pursuit of the end prize. When Lord Sugar tells them they're fired, I take it as meaning they're fired from the process.

Is this why they never have employment lawyers on?

"You're Fired!"
"Well, actually, Lord Shugs, you haven't issued any causes for concern or give me the appropriate period of notice, so I'll just sit back down, thank you very much. Or I could leave and sue you for wrongful dismissal."