In short, because the scan has simply not completed yet.
It all depends on how many files are on the system and if you have any compressed files and how big they are. Depending on these factors, it can take a long time.
In your AV policy, you can set a scan duration for scheduled scans. This means the scan will only run for up to "x" hours, depending on what you set. here.
Go to your AV policy and select your scheduled scan. On the "Schedule" tab edit the setting for "Scan for up to "x" hours.
This is only applicable for scheduled scans though, not manual.
If you want to use this option, you will need to use a scheduled scan instead.
A helpful article to read is here:
Adjusting scans to improve computer performance