The most obvious reason for major developers to ignore retargeting practise is probably very banal — they are used to work in a certain manner. But it's certainly not the only reason.
Setting up retargeting activity is not an easy task. There's a significant work to be done up-front, such as pooling lists Device IDs tracking platforms, defining proper targeting criteria, form target audiences and setting everything on various ad-serving services. Such process, especially if made manually, will inevitably require significant resources from the developer. On practice, even the simplest task of pulling and forming device ID list, might consume several day from a backend programmer.
More so, such task needs to be done on a regular basis since active users tend to become inactive after a certain time period, and we need to try and re-engage him several times during his life circle within our app or our service.
Advanced advertising platforms, such as Facebook, offer semi-automatic adjustment of retargeting campaigns, with auto updated segments. However it requires quite complicated setup, and many developers usually abandon this practice.
In addition, there's an issue of time-lag before seeing results of such activity. For example, in case of limited number of device IDs available, it will take time to drive traction from relatively small targeted audience due to difficulties of "catching" these users on the web. It will certainly be easier to target wider audience.
In order to achieve optimal results you will need to work with multiple sources, to build extensive ID lists and to make various targeted segments.
If the app is already installed on the retargeted device, we should redirect the user into an internal page within the app by using unique deep-link, making sure he will end up in the most relevant place for re-engaging. But such tactic also requires precise tracking support in order to allow efficient retargeting economics and ROI calculation. For this the developer must have deep understanding of the available tracking platforms (AppsFlyer, MAT, Adjust).
On practice, deep-links don't always work as expected. They might open the app-store instead of the already installed application. This may happen since upon clicking, tracker must determine whether the app is installed on the device, but it does not always work properly.
As a result — campaign is not performing as intended.
Another potential difficulty might be caused by increased complexity of setting up retargeting advertising campaigns. In such setup, instead of standard targeting criteria of gender, age and OS, you need to make complex user segments, taking huge number of variables in consideration. For example, you will prefer to spend less money on a user that has been inactive for a few days, but you should be ready to pay more for a user that has been inactive for a long period.