Some banks are actively promoting them as good investments

“For customers with more than 1 million euros in liquid assets, a classic car can be an attractive addition to their portfolio in terms of yield and value stability,” Jens Berner, vintage car expert at Suedwestbank AG’s asset management unit, said in an interview. “After the financial crisis, requests for alternative investments such as art, wine or classic cars had risen sharply.” 

Basically its just being lumped into “real” property investment