I can think of two reasons not to name a specific number; one ideological and one material.
First the Ideological: They are careful not to name a number lest they leave some cash on the table. They would hate to ask for a trillion dollars and find out Congress was ultimately willing to give 1.5.
The second reason is that they don’t even know how much they really intend to spend. It’s not like they have a budget and know how much the shortfall is. They want to spend whatever they can on whatever they want. Give them $3 trillion, they’ll spend it in 12 months. Give them $5 trillion, they can arrange to spend that too. Add both of those numbers and this administration would be willing to spend all THAT as well.