I guess The Netherlands has plenty of billionairers. But Saudis have trillionaires. As a country they are not as rich surely but on an individual level it's Saudis that control the cash flow everywhere at this point. Up until 10 years they invested like 860 billion dollars in the US only. I doubt any EU country has that power among individuals.
There are two things here. First of all, for reasons i mentioned above, a bigger share of the pie is concentrated at the top in a country like Saudi Arabia. Secondly, when talking about foreign investments. Let's not forget EU countries are accountable to an electorate, and to a tax paying citizenry. Middle Eastern countries are not. ME rulers rule their countries for life and then leave it to their offspring. Their sovereign wealth funds, which are responsible for investing state capital do not answer to anybody. Those factors contribute to making it easier for them to invest significant capital for "soft power" wherever they want.
None of those things buy you a productive and highly developed economy though. At the end of the day, Germany's economy is 4-5 times the size of Saudi Arabia's.