firstly, if users only browse the web, gaming or do some office work, WD or MSE can be extremely light for them because they have no web filter
any AV with a web filter is considered to be heavier than WD in this case, obviously
many AVs have their web filters slowing down web browsing noticeably. With some tweaks and workarounds the speed can be vastly better
if users are producers, coders or they do heavy tasks, WD is not for them because it will lengthen the processing time especially while exporting projects
secondly, people consider something lighter than the others when they have not yet tried lighter AVs (with proper tweaks) or they don't have enough patience to wait until cache collection finishes (which can take a few days, up to 1-2 weeks). The cache generation process can take a lot of resources or disk IO => people uninstall the AV in 1-2 days
after this process, the AV will be much lighter
WD almost doesn't have such caching mechanism or there is but insignificant => can be used straight out-of-the-box