Tldr: Advanced western equipment takes so long to reach Ukraine that Russia adapts to it's potential capabilities before it's deployed.
Relevant section:
Unnamed Ukrainian military officials said there is an unfortunate pattern of the West finally sending weapons when they are "no longer relevant" for the current battlefield dynamics, Politico reported on April 3, citing sources.
While Ukraine's Western allies have provided tens of billions of military aid and equipment, some of the more advanced systems pledged, such as the F-16 fighter jet, have been slow-walked and have yet to reach the battlefield. At the same time, Russia has been steadily adapting its defensive capabilities to counter new Western weapons as they arrive.
An international coalition to train Ukrainian pilots on F-16s was launched in the summer of 2023. Belgium, the Netherlands, Denmark, and Norway have also pledged to supply Ukraine with dozens of U.S.-made fourth-generation jets.
Yet, the unnamed Ukrainian military officials told Politico that the "F-16s were needed in 2023; they won't be right for 2024."
The official explained that Russia has been testing its air defense posture and has likely deployed its radar and anti-air missiles in a manner that would render much of the F-16s' offensive capabilities ineffective.