In 25:3 it says, "וזאת התרומה אשר תקחו מאתם" - this is the donation that you should take from them.
Rashi comments that it was all בנדבה - given generously. If that is the case, why does it keep says "Tikchu" you should take from them? they are donating it? They are coming and giving it to Moshe, so why does he have to go take it?
People make donations, during an appeal in shul for example, but they are lazy about actually getting around to making good on it. The Treasurer has to remind people that they have not yet paid. He has to keep nudging and even to take it from them.
When you are building the Beis Hamikdash, the Mishkan, a Shul, a Tzedaka project, etc., you cannot just sit back and rely on the promises people make as to whether they will donate and how much. If you rely on their promises, your project will never get off the ground.
You have to go get the money. Make them actually pay up. People promise to donate, but when they have to pay, they get reluctant,. It is difficult to pay out.
They donated generously, but you still have to go take it from them.