Left Job – Advice for 401(k) – why rollover into IRA instead of combining 401(k)s?

Left my old employer with 100k+ in 401(k).

Starting new job’s 401(k) at $0.

Most people on the internet seem to suggest it’s best to rollover old 401(k)s into IRAs. Feel like then I’d have one semi-sizeable IRA and a tiny new 401(k). Any reason(s) to instead rollover my old 401(k) into my new 401(k) and just have one main retirement fund? What are the main considerations for each option?

Additional considerations: mid-20s, have a Roth IRA but income not above limit to contribute further. [Bonus question – what should I do with my existing Roth IRA if I expect to always be above the income limit to contribute moving forward?]

Thanks a ton for any tips.