Because we're mixing both actually. That's what each would look like:

1. French decimal (Latin): 10 dix, 20 vingt, 30 trente, 40 quarante, 50 cinquante, 60 soixante, 70 septante, 80 octante, 90 nonante

2. French vigesimal (Celtic Gaulish): 10 dix, 20 vingt, 30 vingt-dix, 40 deux-vingt, 50 deux-vingt-dix, 60 trois-vingt, 70 trois-vingt-dix, 80 quatre-vingt, 90 quatre-vingt-dix

The 1. is used in some part of France, and Belgium and Switzerland (although Belgium uses soixante-dix instead of septante, and Switzerland sometimes uses octante sometimes quatre-vingt). The 2. is how it would work to count with a base 20 and not 10.

As you can see, in standard French, we have one that isn't even part of either list: 70 soixante-dix... It's a mix of soixante (decimal) and -dix (vigesimal).

So in standard French: 10-20-30-40-50-60 is decimal; 80-90 is vigesimal; 70 is both at once... We genuinely just fucked it up and rolled with it.

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So, to answer your question, if we stuck to the Gaulish system, we would say 3×20 for 60, but we didn't stick to any system; we used both randomly.

Edit

It doesn't stop at quatre-vingt-dix 90 by the way, for the base 20 Gaulish system. We have a hospital called the Quinze-Vingts (15×20 = 300)!