Which seems perfectly reasonable considering it's a totally free product, which relies on ad revenue to fund running and development costs?
"lots of effort would go it it" is true regardless of software licensing.
It would be great if all software were free and open-source, but it makes it hard to incentivize people to create that software if they're not being compensated.
Photopea has a paid version, so people who don't want ads can just use that version.