Company A had already interviewed you and liked you enough to offer. If Company B can hire you instead they've possibly gotten a free ride on the "technical qualifications" part of the interview process. The fact that you have an offer means you've already passed a lot of qualification checking from someone.
The fact that you have an offer means you've already passed a lot of qualification checking from someone
But if you already have a job, that’s the same evidence isn’t it? So why does anyone need to do a technical qualification interview after their first job?
If one already has a job means one perhaps passed the technical interview but long time ago and meanwhile could become lazy incompetent bum;) Getting a fresh offer means one is back in shape, a competence somehow transferable it seems. Just my divagation.
...this feels like it might be a bit of a job interview shortcut or lifehack. Just always say you've been given an offer from other companies, and help reduce the amount of crap you have to put up with.