Who is "they"? If it's the case that the first level courts to which these administrative law matters are brought can't hear from anyone the two sides want to bring in and that judges there have no staff to whom to delegate, I stand corrected.
Let me disagree with
@Oldgateboatdriver a bit on this.
Firstly, I'll say that it is highly unusual for a judge to call a witness including expert witnesses but it is not unknown and the ability to do so comes from our common law roots where superior court judges are responsible for the judicial process in the case before them.
Hard to find a Canadian discourse on this but
here's an American one that discusses the basics. There are even some American codes that expressly allow this.
In Canada there are a few SCC cases that touch on this, usually in the criminal context where the crown has failed to call an essential witness, such as the complainant. See, for example,
R v Cook where the following is stated:
63 With regard to Ryan J.A.’s second concern, aside from the comments I have already made, I do not feel a need to expand at length upon the trial judge’s discretion to call witnesses given that this issue was extensively canvassed by the reasons of Cory J. in Finta, supra. As he stated in that case, the calling of witnesses by the trial judge is a matter to be left to each judge’s discretion, and that it should be exercised in rare cases so as to avoid overly interfering with the adversarial nature of the proceedings: Finta, supra, at p. 861. A number of factors, some of which I have already discussed, will have to be considered by the trial judge in determining whether or not to exercise this discretion, but the guiding principle is that in doing so it must be “essential . . . in order to do justice in the case”.
Cook discusses some of the pros and cons of why a specific counsel may decide to not call a given witness.
I would think that especially with expert witnesses (aka SMEs) courts would be very reluctant to call their own expert. This is because it can become an issue that the trial judge has abrogated the decision making process to "his" expert. But, I could conceivably see a case where the judge is left with the conclusion that both the dueling experts are so "bought" by their parties as to be virtually useless. What is more likely, however, is that the judge will look to the issue of whose burden of proof it was to establish the particular expert foundation for the case and hold that the requisite expert has not done so.
All that said, I'm not quite sure of the thrust of the rest of your discussion. There are officers to whom a judge can delegate some work - they're generally called masters or prothonotaries. The degree of delegation relates to many administrative matters but can include substantial decisions. Beyond that court staff is very limited to clerks who manage the files and some who can do research. They do not include experts. Courts are generally lean in the adversarial judicial system relying on the parties.
The big issue about the present USSC decision is exactly the fact that legislatures at all levels have set up administrative bodies with subject matter experts as hearing officers to divert cases from the courts so as to ease the load on both the court and the parties in conducting inquiries. Remember that in the case of administrative bodies, the legislature - acting for the people as a whole - have made laws that give the secretary of a given department the right to set up such agencies and to make regulations that govern them.
Full disclosure: I haven't read the Chevron decision myself. My Give-a-shit factor on what the USSC is doing these days has dropped to below zero. I couldn't tell you whether or not someone has really overstepped their delegated powers or not but when some judges use Originalism as the analytical basis for their decisions I tend to tune out and work on my model railroad. A pox on all their houses.