Injecting mocked dbContext inside Service - integration tests

I'm trying to implement integration test so I can test my endpoints using HttpClient only

public async Task TestEndPoint()
{          
    using (var response = await _client.GetAsync($"/api/car/{id}"))
    {
        //...         
    }
}

I'm using Fixture class to help me with this

private readonly TestServer _server;
public Fixture()
{
    var dbContextOptions = new DbContextOptionsBuilder<CarDbContext>()
                    .UseInMemoryDatabase(Guid.NewGuid().ToString())
                    .Options;

    var mockContext = new Mock<CarDbContext>(dbContextOptions);
    var mockOwnerSet = new Mock<DbSet<Owner>>();
    var mockCarSet = new Mock<DbSet<Car>>();
    mockContext.Setup(m => m.Owners).Returns(mockOwnerSet.Object);
    mockContext.Setup(m => m.Cars).Returns(mockCarSet.Object);          

    var carService = new CarService(mockContext.Object);

    _server = new TestServer(new WebHostBuilder()
        .ConfigureAppConfiguration((context, conf) =>
        {
            conf.AddJsonFile(@Directory.GetCurrentDirectory() + "../appsettings.json");
        }).UseStartup<Startup>()
            .ConfigureServices(services =>
            {
                services.AddDbContext<CarDbContext>(options => options.UseInMemoryDatabase("Test"));
                services.AddScoped<ICarService>(_ => carService);
            })
        );

    Client = _server.CreateClient();    
}   

Inside Startup.cs

public class Startup
{
   public void ConfigureServices(IServiceCollection services)
   {
       services.AddDbContext<CarDbContext>(options => { options.UseSqlServer(Configuration.GetConnectionString("Db")); });
       services.AddTransient<ICarService, CarService>();
   }
}

Service class

public class CarService: ICarService {
    private readonly CarDbContext _carDbContext;

    public CarService(CarDbContext carDbContext)
    {
        _carDbContext = carDbContext;
    }

    public async Task<Car> GetAsync(int id)
    {
        return await _carDbContext.Cars.FindAsync(id); //this always returns null
    }
}

My question is: Why mock db context is not being used inside CarService

return await _carDbContext.Cars.FindAsync(id); always returns null