Why has it taken 4 years and 8 months to find guilty, and sentence, Nikolas Cruz? We know he did it. He plead guilty. He was an adult at the time. He should have been put out of our misery over 4 years ago.
How do you know he did it? Because talking heads on TV told you so? The Central Park 5 all pleaded guilty to charges they did not commit. Have you heard of MKUltra?
The Wikipedia page for the shooting has a fairly detailed sequence of events for the legal process (https://en.wikipedia.org/wiki/Stoneman_Douglas_High_School_shooting#Cruz). It seems the main delays were waiting for his finances to be resolved to determine whether he was due a public defender and then COVID appears to have added at least a year delay. Major criminal cases do tend to take time anyway, however clear-cut you might think the case is, and this is probably fairly standard in the circumstances.