In his second day on the stand of a landmark antitrust trial, Mark Zuckerberg, the chief executive of Meta, said he bought Instagram and WhatsApp because it was hard to build new apps and dodged questions about whether he was trying to snuff out competitive threats to his company.

“Building a new app is hard,” he said Tuesday when asked why, in one 2012 email that was presented, he had seemed intent on buying Instagram. “We’ve probably tried building dozens of apps over the history of the company, and the majority of them don’t go anywhere.”

“We could have built an app,” he added. “Whether it succeeded or not is a matter of speculation.”

Mr. Zuckerberg’s testimony is central to the antitrust trial, being held in the U.S. District Court for the District of Columbia. The chief executive spent several hours on Monday answering questions from lawyers as they tried to make the case that Mr. Zuckerberg saw the other apps as rivals that he needed to take out. During the questioning, which at times became contentious, Mr. Zuckerberg frequently said he didn’t remember his thought process for certain emails.

The case, Federal Trade Commission v. Meta Platforms, poses a consequential threat to Mr. Zuckerberg’s business, which he co-founded as Facebook in his Harvard dorm room in 2004. The F.T.C. is asking Judge James E. Boasberg, who is presiding over the case, to find the company guilty of using a “buy-or-bury” strategy to kill off competition by acquiring nascent rivals like Instagram and WhatsApp. Meta bought Instagram in 2012 for $1 billion and WhatsApp in 2014 for $19 billion.

If successful, the government is likely to ask the judge to break up Meta through selling the two apps.