pretrained object detection models in keras
There are pretrained object recognition models in keras.applications library. But as far as I know, there is no pretrained object detection model available.

Does anyone know why it is the case? Object detection is a big part of problems when dealing with visual problems.

That is because vanilla Keras does not include implementation of methods/models for object detection. 

There are many approaches to object detection with deep learning (see Object Detection with Deep Learning: A Review for a survey), but none of them are implemented as a part of Keras library, so no official models as well. I have a feeling that François Chollet tries to keep it simple and minimalistic, so bloating the code with something like TensorFlow models will be against its philosophy.

However, Keras is easily extendable, so there are plenty of unofficial implementations (e.g. SSD or Mask R-CNN) supplied with the trained models though. See Keras model zoo for more.
