If I remember correctly, it is a combination of multiple reasons. ... More importantly, it acts as a domestic service in order to make money. There were not sufficient cross border journeys for 1 train every 2 hours, so it needs passengers within the Netherlands and within Germany.
More importantly: the route through The Netherlands is quite heavily used so even with less stops it's actually quite hard to save a significant amount of time. Between Amsterdam Central and Amersfoort Central it's almost unavoidable to use the path of the domestic IC service as there is no space left at all between Hilversum and Weesp where 10 passenger trains per hour run on a two track line, which also sees some cargo trains.
East of Amersfoort there is a bit more space, but still an ICE won't be able to run at full speed. And besides that: between the Twente region and Amsterdam the IC-Berlin has also an important domestic function as it's the only Amsterdam Central-bound service from that part of the country (domestic IC-trains run via Amsterdam South and would involve a 15 minute wait to change for Amsterdam Central). Therefore the small amount of doors in an ICE als also an addtional inconvenience.
Besides the heavily used line and the low speed limit there's another factor at play avoiding an increase of the speed limit: the winding line. This is because the electrified route from Apeldoorn to Hengelo historically isn't the main line to the east: the "Oosterspoorweg" historically ran via Zutphen, but as that route serves less important towns and villages it's mainly single track and non-electrified. So not really usefull for an ICE.
Already mentioned were the plans for a fast train using an ICE via Utrecht - Arnhem, but that line is also limited in its residual capacity. The other fast option looks like a detour: via Zwolle to Bad Bentheim. Though it would be faster as the line between Lelystad and Zwolle is equipped with ETCS besides ATB and allows a speed of 200 km/h. However this would require ATB-NG in the ICE to allow routing via the Zwolle - Wierden line, which is also partially single track.