There appears to be various problems :
Various airlines postponed their new aircraft during height of covid, some are built and stored at manufacturer, or at dry locations like Victorville. Some of these were not finished, missing anything from engines to cabin fit out.
Also during covid, airlines offloaded many older planes which are less efficient, and these went for scrap. A lot of BA 747s went to Kemble, Gloucestershire for scrapping. Many of these were due to be withdrawn 2020-23 anyway, but of course withdrawing before the replacements have arrived has left a gap in the fleet.
Boeing had regulatory problems with FAA and other major regulators (as a result of not being 100% clean on the 737-max self certification). Following the 2 crashes the regulators tightened up the checking, and require modifications which are taking long while to do before the planes can enter service. It could be well into 2024 before backlog of modifications done. Some modifications are also being done on wide bodies 787. A spin off of the extra regulatory scrutiny is the new 777X is probably not going to be certified until 3-5 years later than Boeing expected.
Airbus had some problems after cutting capacity during covid. In addition there were airlines not accepting planes eg Qatar. Space requirements meant some of these had to be finished and flown to places like Chateauroux in France, and because many airlines configurations are different isn’t easy to just divert them to those waiting.
Finally there is a staffing issue, many staff left airline industry (and airports) during covid, and currently not enough staff to keep all the planes busy. Heathrow still has a daily cap on number of departing passengers due to lack of staff.