Why did David make lavish preparations for the temple before his death?

Answer: Solomon was still young

David said his son Solomon was young and the LORD's house must be magnificent, so he gathered materials before he died.

“And David said, “My son Solomon is young and inexperienced, and the house to be built for the LORD must be exceedingly magnificent—famous and glorious throughout all lands. Therefore I must make preparations for it.” So David made lavish preparations before his death.”— 1 Chronicles 22:5 (BSB)