Because most of it never left the USA
“Foreign aid” is a giant money laundering scam that goes to fund the NGO/Military political influence machine. In exchange Congress critters get “campaign donations” that are legalized bribery and insider stock trading information.
It may be difficult to believe Zelensky (corruption is a feature of Ukraine's politics since 1991 due to their Soviet Legacy) but we are talking about the Biden Administration, one of the most corrupt Presidential Administrations ever.
So I think Zelensky is being truthful. He didn't want to blow the whistle at the time because Joe Biden wouldn't hesitate to cut him off.