For two reasons.
1) The truth will inevitably earn you a lecture on what you're doing wrong and how this will ruin your life which you'd realize if only you weren't too stupid to see it for yourself but luckily you have parents who are wiser and aren't afraid to offer advice. Best to tell a lie that is close to what they want to hear, but not exactly because that would be too obviously a lie.

and

2) What they don't know, they can't use against you at some future date. It doesn't matter how innocuous you think it is, it will most certainly be used against you in some way at some point. It will come when you least expect it, and it will be in a form you could never have foreseen. Trust me.

So, as i've always advised my younger sister, LIE.

Next week's lunar new year is China's biggest festival. It can also be a major headache for those returning home without a potential spouse. Pressure on young adults to settle down goes into overdrive, as gathering family members begin the inquisition and line up possible candidates.

Taking a boyfriend or girlfriend home is a fast way to curb the speculation, which is why Li, like other twentysomethings, has hired a fake partner through an online agency.

...
"I don't need him to stay long, just one night, New Year's Eve, and he can just say work is busy and he has to go back the next day, like [the guy I hired] last year," she said.

There's always the chance you might be getting involved with someone untrustworthy who will turn into a stalker or a burglar or something. But when faced with the alternative - a weekend spent being made to feel guilty for upsetting your parents as well as having all your shortcomings discussed by the entire family loudly and repeatedly - i can see why they would take the chance and shell out the cash.

And, hey, is it such a strange idea? I'm sure there are plenty of people out there who asked a friend to pretend to be their SO at a wedding or reunion their ex was also attending. It's a fairly common movie theme. There's The Wedding Date, a movie about a woman who hires a male escort to play he boyfriend at her sister's wedding.

And let's not forget the lengths the characters in The Wedding Banquet go to keep the main character's parents from discovering he's gay. As we all know, if it's in a movie or on tv, it must be true.