I agree that investing on these may seem more profitable in the short term. But branding is a long term goal. Even for Startups branding is important, this can be crucial to close the next deals with VCs or acquiring users' hearts.
Digital Ocean is a good example, I've read good and bad stories, mostly good and they had an extensive Youtube and Display Campaigns. When it was finally time for me to close a deal with a VPS to replace my old Dreamhost shared server I didn't have to search. I went straight to Digital Ocean.
This is harder to measure, or even impossible when you add the fact that I was probably impacted at least 50 times by passive or active advertisement for them on at least 4 different devices.
Also keep in mind that I was aware of this the whole time, and I smiled every tie I saw a DO ad, because I knew I was probably gonna sign at some point. But most of the time these type of advertisements happen without you even noticing, even if you are in this business.
Companies spend millions in SuperBowl ads, and nobody can click on that, it's impossible to tell how many coca-cola bottles will be sold by that ad and post discussions that it sparked. But they will do it again next year.
Display advertisement is valuable but it's harder to put a price tag. Even if you don't click on the ad in a mobile app it's still valuable. I'd argue that if you make it annoying or if you try to steal a click it's less valuable.
There is a reason this type of publicity is sold by impressions, because that's where the value is, clicks can or cannot happen, that's just a plus you won't get on TV.