L-cancelled frame advantage
Nair first hit: -4
Nair second hit: -1
Up air tipper: ±0
Up air non-tipper: -1
Fair tipper: ±0
Fair non-tipper: -1
Dair tipper: -9
Dair non-tipper: -10
Bair tipper: -5
Bair non-tipper: -6
Being aggressive doesn't mean you should necessarily hit shields a lot.
With Marth, actually hitting a shield doesn't accomplish much of use; it's not like with Fox where you have strong mixups after hitting a shield with an aerial (which allow you to continue the pressure and are generally high-reward). An opponent is generally relatively safe after shielding a Marth aerial; the powerful part of the aerial on shield isn't the aerial itself but the threat of it hitting them forcing them to shield in the first place, which makes them vulnerable to grab and limits their movement (and many of their options in general).
Once you've actually thrown out your aerial on shield you commit to the lag and eliminate much of the advantage that you had (in exchange for a weak mixup). When you commit to something like that you want it to be worthwhile, and with Marth that's rarely the case. Before you commit you maintain the threat of potentially being able to do that, and also maintain access to whatever other options you could use from your free-to-act state (and the threat of them).
Compare the effectiveness of these somewhat hyperbolic and oversimplified examples:
1) Throw out a fair in neutral, which the opponent shields because they know it's coming on account of you swinging at them often when you have the opportunity. Then probably just have them wavedash/jump/whatever out of shield and reset to neutral since you have little frame advantage or good fast moves to use after the hit (everything you can do which would catch their escape option is either low reward, such as jab or down tilt, or high commitment/unsafe, such as forward smash or rising fair).
2) Move near them without throwing out a fair, making them shield because their shield beats fair, and you've conditioned them to respect your fair by successfully doing something with it (like catching out approaches or backwards movement) on other occasions in that situation or similar ones, or because they already respect it based on previous knowledge of you or the matchup in general. Then be able to grab them out of their shield (which you made them do).
This document contains lots of useful/applicable information (including links to other sources):
https://docs.google.com/document/d/1Daf_Dqod1HoMj7L26ITeUM-xKUKhfxh2vJLWOl2N0C8/edit?pli=1