Because getting stuff now is almost always better than getting stuff later. If you plan to play +200 turns, not harvesting might be the better option over the extreme long term; but, the chances are that getting stuff now is actually the better investment over the long term.
So, for starters, let's say you hard-build a monument, granary and a watermill (with two rice nearby) in that order. Let's say you're making 6 production a turn (you are building basic infrastructure anyways). That means you'll complete the monument on turn +10, the granary on turn +21, and the watermill on turn +35. Another way to think of that is that's: -20 culture, -2 housing, -40 food, -10 production for 10 turns; -20 culture, -2 housing, -84 food, -21 production for 21 turns; and finally total "loss" of -20 culture, -2 housing (for 21 turns), -126 food, and -35 production by the time all three buildings are completed.
This is what is known as an opportunity cost. It's the cost of NOT doing something. In this case, it's the cost of not putting Magnus in a city and not chopping a woods and a jungle. This is actually a bit of a lazy calculation because 126 food is probably enough to get another pop -- who can work whatever tile you want so you'd have to add those yields as well.
Lastly, there's the opportunity cost of time as well. If you chop those three buildings up on turn 0, +1, and +2, that leaves you 33 turns to make whatever else you feel like making WHILE STILL enjoying the benefits of those infrastructure yields. Also note that chopping can be done on the current turn (turn 0), so it gives you a one turn advantage (in chess this is called a tempo). The opportunity cost of time is the largest benefit of chopping. You don't win the game by building infrastructure. Infrastructure is an investment that balloons overtime so that you can produce things that will help you win the game like units or districts or buildings in districts. In other words, infrastructure is like the startup capital for a business. It's necessary to get things moving, but the business itself can't survive on startup capital. It needs to leverage that startup capital into profits (IE - winning the game).